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Convex 
Optimization 
Muhammad Adil 
Raja 
Introduction 
Convex Sets 
Convex 
Functions 
Convex 
Optimization 
Problems 
References 
CONVEX OPTIMIZATION 
Muhammad Adil Raja 
Roaming Researchers, Inc. 
August 31, 2014
Convex 
Optimization 
Muhammad Adil 
Raja 
Introduction 
Convex Sets 
Convex 
Functions 
Convex 
Optimization 
Problems 
References 
OUTLINE 
INTRODUCTION 
CONVEX SETS 
CONVEX FUNCTIONS 
CONVEX OPTIMIZATION PROBLEMS 
REFERENCES
Convex 
Optimization 
Muhammad Adil 
Raja 
Introduction 
Convex Sets 
Convex 
Functions 
Convex 
Optimization 
Problems 
References 
OUTLINE 
INTRODUCTION 
CONVEX SETS 
CONVEX FUNCTIONS 
CONVEX OPTIMIZATION PROBLEMS 
REFERENCES
Convex 
Optimization 
Muhammad Adil 
Raja 
Introduction 
Convex Sets 
Convex 
Functions 
Convex 
Optimization 
Problems 
References 
OUTLINE 
INTRODUCTION 
CONVEX SETS 
CONVEX FUNCTIONS 
CONVEX OPTIMIZATION PROBLEMS 
REFERENCES
Convex 
Optimization 
Muhammad Adil 
Raja 
Introduction 
Convex Sets 
Convex 
Functions 
Convex 
Optimization 
Problems 
References 
OUTLINE 
INTRODUCTION 
CONVEX SETS 
CONVEX FUNCTIONS 
CONVEX OPTIMIZATION PROBLEMS 
REFERENCES
Convex 
Optimization 
Muhammad Adil 
Raja 
Introduction 
Convex Sets 
Convex 
Functions 
Convex 
Optimization 
Problems 
References 
OUTLINE 
INTRODUCTION 
CONVEX SETS 
CONVEX FUNCTIONS 
CONVEX OPTIMIZATION PROBLEMS 
REFERENCES
Convex 
Optimization 
Muhammad Adil 
Raja 
Introduction 
Convex Sets 
Convex 
Functions 
Convex 
Optimization 
Problems 
References 
INTRODUCTION I 
I Many situations arise in machine learning where we 
would like to optimize the value of some function. 
I That is, given a function f : Rn ! R, we want to find 
x 2 Rn that minimizes (or maximizes) f (x). 
I In the general case, finding the global optimum of a 
function can be a very difficult task. 
I However, for a special class of optimization 
problems, known as convex optimization problems, 
we can efficiently find the global solution in many 
cases. 
I “efficiently” has both practical and theoretical 
connotations: 
1. It means that we can solve many real-world problems 
in a reasonable amount of time.
Convex 
Optimization 
Muhammad Adil 
Raja 
Introduction 
Convex Sets 
Convex 
Functions 
Convex 
Optimization 
Problems 
References 
INTRODUCTION II 
2. And it means that theoretically we can solve 
problems in time that depends only polynomially on 
the problem size. 
I The goal of these section notes and the 
accompanying lecture is to give a very brief overview 
of the field of convex optimization.
Convex 
Optimization 
Muhammad Adil 
Raja 
Introduction 
Convex Sets 
Convex 
Functions 
Convex 
Optimization 
Problems 
References 
CONVEX SETS I 
DEFINITION 
A set C is convex if, for any x, y 2 C and  2  with 
0    1, x + (1  )y 2 C. 
I Intuitively, this means that if we take any two 
elements in C, and draw a line segment between 
these two elements, then every point on that line 
segment also belongs to C. 
I Figure 1 shows an example of one convex and one 
non-convex set. 
I The point x + (1  )y is called a convex 
combination of the points x and y.
Convex 
Optimization 
Muhammad Adil 
Raja 
Introduction 
Convex Sets 
Convex 
Functions 
Convex 
Optimization 
Problems 
References 
CONVEX SETS II 
(a) (b) 
Figure 1: Examples of a convex set (a) and a non-convex set (b). 
FIGURE: Examples of a convex set (a) and a non-convex set. 
Examples 
All of Rn. It should be fairly obvious that given any x, y ! Rn, !x + (1 − !)y ! Rn. 
The non-negative orthant, Rn 
Examples 
I All of Rn. It should be fairly obvious that given any 
+. The non-negative orthant consists of all vectors in 
Rn whose elements are all non-negative: Rn 
+ = {x : xi # 0 $i = 1, . . . , n}. To show 
x; y 2 n; x + (1  )y 2 n. 
I The non-negative orthant, n 
that this is a convex set, simply note that given any x, y ! Rn 
+ and 0 % ! % 1, 
+. The non-negative 
(!x + (1 − !)y)i = !xi + (1 − !)yi # 0 $i. 
orthant consists of all vectors in n whose elements 
are all non-negative: 
Norm balls. Let  ·  be some norm on Rn (e.g., ! the Euclidean norm, x2 = n 
x2i 
). Then the set {x : x % 1} is a convex set. To see this, suppose x, y ! Rn,
Convex 
Optimization 
Muhammad Adil 
Raja 
Introduction 
Convex Sets 
Convex 
Functions 
Convex 
Optimization 
Problems 
References 
CONVEX SETS III 
1. n 
+ = x : xi  08i = 1; :::; n. 
2. To show that this is a convex set, simply note that 
given any x; y8Rn+ 
and 0 ?  1, 
(x + (1  )y)i = xi + (1  )yi  08i. 
I Norm balls. Let jj:jj be some norm on n (e.g., the 
Euclidean norm, x2 = 
qPni 
=1 x2 
i ). Then the set 
x : jjxjj  1 is a convex set. 
I To see this,suppose x; y 2 Rn, with 
jjxjj  1; jjyjj  1,and 0    1. Then 
jjx + (1  )yjj  jjxjj + jj(1  )yjj = 
jjxjj + (1  )jjyjj  1 
where we used the triangle inequality and the 
positive homogeneity of norms.
Convex 
Optimization 
Muhammad Adil 
Raja 
Introduction 
Convex Sets 
Convex 
Functions 
Convex 
Optimization 
Problems 
References 
CONVEX SETS IV 
I Affine subspaces and polyhedra. Given a matrix 
A 2 mn and a vector b 2 m, an affine subspace is 
the set x 2 n : Ax = b (note that this could possibly 
be empty if b is not in the range of A). Similarly, a 
polyhedron is the (again, possibly empty) set 
x 2 Rn : Ax  b, where “” here denotes 
componentwise inequality (i.e., all the entries of Ax 
are less than or equal to their corresponding element 
in b). To prove this, first consider x; y 2 n such that 
Ax = Ay = b. Then for 0    1, 
A(x+(1)y) = Ax+(1)Ay = b+(1?)b = b. 
Similarly,for x; y 2 n that satisfy Ax  b andAy  b 
and 0    1, 
A(x+(1)y) = Ax+(1)Ay  b+(1)b = b.
Convex 
Optimization 
Muhammad Adil 
Raja 
Introduction 
Convex Sets 
Convex 
Functions 
Convex 
Optimization 
Problems 
References 
CONVEX SETS V 
I Intersections of convex sets. Suppose 
CT1,C2,...,Ck are convex sets. Then their intersection ki 
Ci kix x Cii 1k is also a convex set. 
=1 = : 2 8= ; :::; TTo see this, consider x; y 2 
1 Ci and 0 =?  1. 
ki 
Then, x + (1  )y 2 Ci8i = 1; :::; k by the Tdefinition 
of a convex set. Therefore x + (1  )y 2 
=1 Ci . 
Note, however, that the union of convex sets in 
general will not be convex. 
I Positive semidefinite matrices. The set of all 
symmetric positive semidefinite matrices, often times 
called the positive semidefinite cone and denoted 
Sn+ 
, is a convex set (in general, Sn  nn denotes 
the set of symmetric n ? n matrices). Recall that a 
matrix A 2 nn is symmetric positive semidefinite if 
and only if A = AT and for all x 2 Rn, xT Ax  0. Now 
consider two symmetric positive semidefinite
Convex 
Optimization 
Muhammad Adil 
Raja 
Introduction 
Convex Sets 
Convex 
Functions 
Convex 
Optimization 
Problems 
References 
CONVEX SETS VI 
matrices A;B 2 Sn+ 
and0    1. Then for any 
x 2 n, 
xT (A + (1  )B)x = xT Ax + (1  )xT Bx  0. 
The same logic can be used to show that the sets of 
all positive definite, negative definite, and negative 
semidefinite matrices are each also convex.
Convex 
Optimization 
Muhammad Adil 
Raja 
Introduction 
Convex Sets 
Convex 
Functions 
Convex 
Optimization 
Problems 
References 
CONVEX FUNCTIONS I 
DEFINITION 
A function f : n !  is convex if its domain (denoted 
D(f)) is a convex set, and if, for all x; y 2 D(f ) and 
 2 R; 0 ?  1, f (x + (1  )y)  f (x) + (1  )f (y). 
I Intuitively, the way to think about this definition is that 
if we pick any two points on the graph of a convex 
function and draw a straight line between them, then 
the portion of the function between these two points 
will lie below this straight line. This situation is 
pictured in Figure 2. 
I We say a function is strictly convex if Definition 2 
holds with strict inequality for x?6= y and 0    1. 
We say that f is concave if – f is convex, and likewise 
that f is strictly concave if – f is strictly convex.
Convex 
Optimization 
Muhammad Adil 
Raja 
Introduction 
Convex Sets 
Convex 
Functions 
Convex 
Optimization 
Problems 
References 
CONVEX FUNCTIONS II 
convex FIGURE: function. Graph of a convex By function. the definition By the definition of of 
convex functions, the graph must lie above the function. 
convex functions, the line connecting two points on the graph 
must lie above the function. 
Condition for Convexity
Convex 
Optimization 
Muhammad Adil 
Raja 
Introduction 
Convex Sets 
Convex 
Functions 
Convex 
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Problems 
References 
FIRST ORDER CONDITION FOR CONVEXITY I 
I Suppose a function f : n !  is differentiable. 
I The f is convex if and only if D(f ) is a convex set and 
for all x; y 2 D(f ), f (y)  f (x) + 5x f (x)T (y  x): 
I The function 5x f (x)T (y x): is called the first-order 
approximation to the function f at the point x. 
I Intuitively, this can be thought of as approximating f 
with its tangent line at the point x. 
I The first order condition for convexity says that f is 
convex if and only if the tangent line is a global 
underestimator of the function f . 
I In other words, if we take our function and draw a 
tangent line at any point, then every point on this line 
will lie below the corresponding point on f . 
I A function is differentiable if the gradient 5f (x) exists 
at all points x in the domain of f .
Convex 
Optimization 
Muhammad Adil 
Raja 
Introduction 
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FIRST ORDER CONDITION FOR CONVEXITY II 
I The gradient is defined as 
5x f (x) 2 n; (5x f (x))i = @f (x) 
@(x)i 
. 
I Similar to the definition of convexity, f will be strictly 
convex if this holds with strict inequality, concave if 
the inequality is reversed, and strictly concave if the 
reverse inequality is strict.
corresponding point on f. 
the definition of convexity, f will be strictly convex if this holds Convex 
with Optimization 
concave if the inequality is reversed, and strictly concave if the Muhammad reverse Adil 
inequality 
Raja 
Introduction 
Convex Sets 
Convex 
Functions 
Convex 
Optimization 
Problems 
References 
FIRST ORDER CONDITION FOR CONVEXITY III 
Figure 3: Illustration of first-order condition for convexity. 
the gradient is defined as xf(x) # Rn, (xf(x))i = !f(x) 
. For a review on gradients !xi 
previous section notes on linear algebra. 
FIGURE: Illustration od the first-order condition for convexity.
Convex 
Optimization 
Muhammad Adil 
Raja 
Introduction 
Convex Sets 
Convex 
Functions 
Convex 
Optimization 
Problems 
References 
SECOND ORDER CONDITION FOR CONVEXITY 
I 
2x 
I Suppose a function f : n !  is twice differentiable. 
I Then f is convex if and only if D(f ) is a convex set 
and its Hessian is positive semidefinite: i.e., for any 
x 2 D(f ), 5f (x)  0. 
I Here, the notation “” when used in conjunction with 
matrices refers to positive semidefiniteness, rather 
than componentwise inequality. 
I In one dimension, this is equivalent to the condition 
that the second derivative f 00(x) always be positive 
(i.e., the function always has positive curvature).
Convex 
Optimization 
Muhammad Adil 
Raja 
Introduction 
Convex Sets 
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Convex 
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SECOND ORDER CONDITION FOR CONVEXITY 
II 
2x 
I Again analogous to both the definition and first order 
conditions for convexity, f is strictly convex if its 
Hessian is positive definite, concave if the Hessian is 
negative semidefinite, and strictly concave if the 
Hessian is negative definite. 
I A function is twice differentiable if the Hessian 
5f (x) is defined for all points x in the domain of f . 
2x 
I The Hessian is defined as 
5f (x) 2 nn; 2x 
(5f (x))ij = @2f (x) 
@xi@xj 
. 
I For a symmetric matrix X 2 Sn;X  0 denotes that 
X is negative semidefinite. 
I As with inequalities, “” and “”.
Convex 
Optimization 
Muhammad Adil 
Raja 
Introduction 
Convex Sets 
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JENSEN’S INEQUALITY I 
I Suppose we start with the inequality in the basic 
definition of a convex function 
f (x + (1  )y)  f (x) + (1  )f (y)for0    1. 
I Using induction, this can be fairly easily extended to 
convex combinations of more than one point, 
f ( 
Pki 
=1 ixi )  
Pki 
=1 i f (xi ) for 
Pki 
=1 i = 1; i  08i. 
I In fact, this can also be extended to infinite sums or 
integrals. 
I In the latter case, the inequality can be written as 
f ( 
R 
p(x)xdx)  
R 
R p(x)f (x)dx for 
p(x)dx = 1; p(x)  08x. 
I Because p(x) integrates to 1, it is common to 
consider it as a probability density, in which case the 
previous equation can be written in terms of 
expectations, f (E[x])  E[f (x)]. 
I This last inequality is known as JensenÕs inequality.
Convex 
Optimization 
Muhammad Adil 
Raja 
Introduction 
Convex Sets 
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Convex 
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References 
SUBLEVEL SETS I 
I Convex functions give rise to a particularly important 
type of convex set called an -sublevel set. 
I Given a convex function f : n !  and a real 
number  2 , the -sublevel set is defined as 
x 2 D(f ) : f (x)  . 
I In other words, the -sublevel set is the set of all 
points x such that f (x)  . 
I To show that this is a convex set, consider any 
x; y 2 D(f ) such that f (x)   and f (y)  . 
I Then f (x + (1  )y)  f (x) + (1  )f (y)  
 + (1  ) = .
Convex 
Optimization 
Muhammad Adil 
Raja 
Introduction 
Convex Sets 
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EXAMPLES OF CONVEX FUNCTIONS I 
I We begin with a few simple examples of convex 
functions of one variable, then move on to 
multivariate functions. 
I Exponential. Let f :  ! ; f (x) = eax for any 
a 2 . To show f is convex, we can simply take the 
second derivative f 00(x) = a2eax , which is positive for 
all x. 
I Negative logarithm. Let f :  ! ; f (x) =?logx with 
domain D(f ) = R++. 
I Here, R++ denotes the set of strictly positive real 
numbers, x : x  0. 
I Then f 00(x) = 1=x2  0 for all x. 
I Affine functions. Let f : n ! ; f (x) = bT x + c for 
some b 2 n, c 2 . 
I In this case the Hessian, 52xf (x) = 0 for all x.
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Muhammad Adil 
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Introduction 
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EXAMPLES OF CONVEX FUNCTIONS II 
12 
I Because the zero matrix is both positive semidefinite 
and negative semidefinite, f is both convex and 
concave. 
I In fact, affine functions of this form are the only 
functions that are both convex and concave. 
I Quadratic functions. Let 
f : n ! ; f (x) = xT Ax + bT x + c for a symmetric 
matrix A 2 Sn, b 2 n and c 2 . 
I The Hessian for this function is given by 
52xf (x) = A. 
I Therefore, the convexity or non-convexity of f is 
determined entirely by whether or not A is positive 
semidefinite: if A is positive semidefinite then the 
function is convex (and analogously for strictly 
convex, concave, strictly concave).
Convex 
Optimization 
Muhammad Adil 
Raja 
Introduction 
Convex Sets 
Convex 
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Optimization 
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EXAMPLES OF CONVEX FUNCTIONS III 
22 
I If A is indefinite then f is neither convex nor concave. 
I Note that the squared Euclidean norm 
f (x) = jjxjj= xT x is a special case of quadratic 
ni 
functions where A = I; b = 0; c = 0, so it is therefore 
a strictly convex function. 
I Norms. Let f ;n !  be some norm on n. Then by 
the triangle inequality and positive homogeniety of 
norms, for x; y 2 n; 0    1, f (x + (1  )y)  f (x) + f (1  )y) = f (x) + (1  )f (y). 
I This is an example of a convex function where it is 
not possible to prove convexity based on the second 
or first order conditions. 
I The reason is that because norms are not generally 
differentiable Peverywhere (e.g., the 1-norm, 
jjxjj1 = 
=1 jxi j, is non-differentiable at all points 
where any xi is equal to zero).
Convex 
Optimization 
Muhammad Adil 
Raja 
Introduction 
Convex Sets 
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Convex 
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References 
EXAMPLES OF CONVEX FUNCTIONS IV 
I Nonnegative weighted sums of convex functions. 
Let f1; f2; :::; fk be convex functions and w1;w2; :::;wk 
be nonnegative real numbers. Then 
f (x) = 
Pki 
=1 wi fi (x) is a convex function, since 
f (x + (1?)y) = 
Pki 
=1 wi fi (x + (1  )y) (1) 
 
Pki 
=1 wi (fi (x) + (1  )fi (y)) (2) 
=  
Pki 
=1 wi fi (x) + (1  ) 
Pki 
=1 wi fi f (y)(3) 
= f (x) + (1  )f (x) (4)
Convex 
Optimization 
Muhammad Adil 
Raja 
Introduction 
Convex Sets 
Convex 
Functions 
Convex 
Optimization 
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References 
CONVEX OPTIMIZATION PROBLEMS I 
I Armed with the definitions of convex functions and 
sets, we are now equipped to consider convex 
optimization problems. 
I Formally, a convex optimization problem in an 
optimization problem of the form minimize f (x) 
subject to x 2 C. 
I where f is a convex function, C is a convex set, and 
x is the optimization variable. 
I However, since this can be a little bit vague, we often 
write it often written as minimize f (x) subject to 
gi (x)  0, i = 1; :::;m hi (x) = 0; i = 1; :::; p 
I where f is a convex function, gi are convex functions, 
and hi are affine functions, and x is the optimization 
variable.
Convex 
Optimization 
Muhammad Adil 
Raja 
Introduction 
Convex Sets 
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CONVEX OPTIMIZATION PROBLEMS II 
I It is important to note the direction of these 
inequalities: 
1. A convex function gi must be less than zero. 
2. This is because the 0-sublevel set of gi is a convex 
set, so the feasible region, which is the intersection 
of many convex sets, is also convex (recall that affine 
subspaces are convex sets as well). 
3. If we were to require that gi  0 for some convex gi , 
the feasible region would no longer be a convex set, 
and the algorithms we apply for solving these 
problems would not longer be guaranteed to find the 
global optimum. 
4. Also notice that only affine functions are allowed to 
be equality constraints. 
5. Intuitively, you can think of this as being due to the 
fact that an equality constraint is equivalent to the 
two inequalities hi  0 and hi  0.
Convex 
Optimization 
Muhammad Adil 
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Introduction 
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CONVEX OPTIMIZATION PROBLEMS III 
6. However, these will both be valid constraints if and 
only if hi is both convex and concave, i.e., hi must be 
affine. 
I The optimal value of an optimization problem is 
denoted p (or sometimes f ) and is equal to the 
minimum possible value of the objective function in 
the feasible region. p = 
minf (x) : gi (x)  0; i = 1; :::;m; hi (x) = 0; i = 1; :::; p. 
I We allow p to take on the values +1 and 1 when 
the problem is either infeasible (the feasible region is 
empty) or unbounded below (there exists feasible 
points such that f (x) ! 1), respectively. 
I We say that x is an optimal point if f (x?) = p. 
I Note that there can be more than one optimal point, 
even when the optimal value is finite.
Convex 
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Muhammad Adil 
Raja 
Introduction 
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GLOBAL OPTIMALITY IN CONVEX PROBLEMS I 
I Defining the concepts of local optima and global 
optima are in order. 
I Intuitively, a feasible point is called locally optimal if 
there are no “nearby” feasible points that have a 
lower objective value. 
I Similarly, a feasible point is called globally optimal if 
there are no feasible points at all that have a lower 
objective value. 
I To formalize this a little bit more, following definitions 
are important.
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GLOBAL OPTIMALITY IN CONVEX PROBLEMS 
II 
DEFINITION 
A point x is locally optimal if it is feasible (i.e., it satisfies 
the constraints of the optimization problem) and if there 
exists some R  0 such that all feasible points z with 
jjx  zjj2  R, satisfy f (x)  f (z). 
DEFINITION 
A point x is globally optimal if it is feasible and for all 
feasible points z, f (x)  f (z). 
I We now come to the crucial element of convex 
optimization problems, from which they derive most 
of their utility.
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GLOBAL OPTIMALITY IN CONVEX PROBLEMS 
III 
I The key idea is that for a convex optimization 
problem all locally optimal points are globally optimal. 
I LetÕs give a quick proof of this property by 
contradiction. 
I Suppose that x is a locally optimal point which is not 
globally optimal, i.e., there exists a feasible point y 
such that f (x)  f (y). 
I By the definition of local optimality, there exist no 
feasible points z such that 
jjx?zjj2  Randf (z)  f (x).
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GLOBAL OPTIMALITY IN CONVEX PROBLEMS 
IV 
I But now suppose we choose the point 
z = y + (1  )x with  = R2 
jjx?yjj2 Then 
jjx  zjj2 = jjx  ( R 
2jjxyjj2 
y + (1  R 
2jjxyjj2)x)jj2 
(5) 
= jj R 
2jjxyjj2 
(x  y)jj2 (6) 
= R=2  R: (7) 
I In addition, by the convexity of f we have 
f (z) = f (y + (1?)x)  f (y) + (1?)f (x)  f (x). 
I Furthermore, since the feasible set is a convex set, 
and since x and y are both feasible z = y + (1?) 
will be feasible as well. 
I Therefore, z is a feasible point, with jx?zj2  R and 
f (z)  f (x).
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GLOBAL OPTIMALITY IN CONVEX PROBLEMS 
V 
I This contradicts our assumption, showing that x 
cannot be locally optimal.
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SPECIAL CASES OF CONVEX PROBLEMS I 
I For a variety of reasons, it is often times convenient 
to consider special cases of the general convex 
programming formulation. 
I For these special cases we can often devise 
extremely efficient algorithms that can solve very 
large problems, and because of this you will probably 
see these special cases referred to any time people 
use convex optimization techniques. 
I Linear Programming. We say that a convex 
optimization problem is a linear program (LP) if both 
the objective function f and inequality constraints gi 
are affine functions. 
I In other words, these problems have the form 
minimize cT x + d subject toGx  h.
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SPECIAL CASES OF CONVEX PROBLEMS II 
I Ax = b where x 2 n is the optimization variable, 
c 2 n, d 2 , G 2 mn, h 2 m, A 2 p?n, b 2 Rp 
are defined by the problem, and “” denotes 
elementwise inequality. 
I Quadratic Programming. We say that a convex 
optimization problem is a quadratic program (QP) if 
the inequality constraints gi are still all affine, but if 
the objective function f is a convex quadratic 
function. 
I In other words, these problems have the form, 
minimize 1 
2xT Px + cT x + d subject to Gx  h 
+ 
Ax = nb. where again x 2 n is the optimization 
variable, c 2 n, d 2 , G 2 mn, h 2 Rm, 
A 2 pn, b 2 Rp are defined by the problem, but we 
also have P 2 S, a symmetric positive semidefinite 
matrix.
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SPECIAL CASES OF CONVEX PROBLEMS III 
I Quadratically Constrained Quadratic 
Programming. We say that a convex optimization 
problem is a quadratically constrained quadratic 
program (QCQP) if both the objective f and the 
inequality constraints gi are convex quadratic 
functions, minimize 1 
2xT Px + cT x + d subject to 
1 
2xTQix + r T 
i x + si  0; i = 1; :::;m Ax = b where, as 
before, x 2 n is the optimization variable, c 2 n, 
d 2 , A 2 pn, b 2 p, P 2 Sn+ 
, but we also have 
Qi 2 Sn+ 
; ri 2 n; si 2  ,for i = 1,...,m. 
I Semidefinite Programming has become more and 
more prevalent in many different areas of machine 
learning research, so you might encounter these at 
some point, and it is good to have an idea of what 
they are.
Convex 
Optimization 
Muhammad Adil 
Raja 
Introduction 
Convex Sets 
Convex 
Functions 
Convex 
Optimization 
Problems 
References 
SPECIAL CASES OF CONVEX PROBLEMS IV 
I We say that a convex optimization problem is a 
semidefinite program (SDP) if it is of the form 
minimizetr (CX) subject to tr (AiX) = bi , i = 1; :::; p 
X  0. 
I where the symmetric matrix X 2 Sn is the 
optimization variable, the symmetric matrices C, 
A1; :::; Ap 2 Sn are defined by the problem, and the 
constraint X  0 means that we are constraining X 
to be positive semidefinite. 
I This looks a bit different than the problems we have 
seen previously, since the optimization variable is 
now a matrix instead of a vector.
Convex 
Optimization 
Muhammad Adil 
Raja 
Introduction 
Convex Sets 
Convex 
Functions 
Convex 
Optimization 
Problems 
References 
SPECIAL CASES OF CONVEX PROBLEMS V 
I It should be fairly obvious from the definitions that 
quadratic programs are more general than linear 
programs (since a linear program is just a special 
case of a quadratic program where P = 0), and 
likewise that quadratically constrained quadratic 
programs are more general than quadratic programs. 
I However, what is not obvious at all is that 
semidefinite programs are in fact more general than 
all the previous types. 
I That is, any quadratically constrained quadratic 
program (and hence any quadratic program or linear 
program) can be expressed as a semidefinte 
program. 
I
Convex 
Optimization 
Muhammad Adil 
Raja 
Introduction 
Convex Sets 
Convex 
Functions 
Convex 
Optimization 
Problems 
References 
EXAMPLES OF CONVEX OPTIMIZATION I 
I Support Vector Machines. 
I Constrained Least Squares. 
I Maximum likelihood for Logistic Regression.
Convex 
Optimization 
Muhammad Adil 
Raja 
Introduction 
Convex Sets 
Convex 
Functions 
Convex 
Optimization 
Problems 
References 
REFERENCES I 
I The images and inspiration for preparing these slides 
have been taken from Andrew Ngs course on 
Machine Learning. 
I The course is available on Stanford engineering for 
all and Coursera.

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Work-Permit-Receiver-in-Saudi-Aramco.pptx
 

Convex Optimization

  • 1. Convex Optimization Muhammad Adil Raja Introduction Convex Sets Convex Functions Convex Optimization Problems References CONVEX OPTIMIZATION Muhammad Adil Raja Roaming Researchers, Inc. August 31, 2014
  • 2. Convex Optimization Muhammad Adil Raja Introduction Convex Sets Convex Functions Convex Optimization Problems References OUTLINE INTRODUCTION CONVEX SETS CONVEX FUNCTIONS CONVEX OPTIMIZATION PROBLEMS REFERENCES
  • 3. Convex Optimization Muhammad Adil Raja Introduction Convex Sets Convex Functions Convex Optimization Problems References OUTLINE INTRODUCTION CONVEX SETS CONVEX FUNCTIONS CONVEX OPTIMIZATION PROBLEMS REFERENCES
  • 4. Convex Optimization Muhammad Adil Raja Introduction Convex Sets Convex Functions Convex Optimization Problems References OUTLINE INTRODUCTION CONVEX SETS CONVEX FUNCTIONS CONVEX OPTIMIZATION PROBLEMS REFERENCES
  • 5. Convex Optimization Muhammad Adil Raja Introduction Convex Sets Convex Functions Convex Optimization Problems References OUTLINE INTRODUCTION CONVEX SETS CONVEX FUNCTIONS CONVEX OPTIMIZATION PROBLEMS REFERENCES
  • 6. Convex Optimization Muhammad Adil Raja Introduction Convex Sets Convex Functions Convex Optimization Problems References OUTLINE INTRODUCTION CONVEX SETS CONVEX FUNCTIONS CONVEX OPTIMIZATION PROBLEMS REFERENCES
  • 7. Convex Optimization Muhammad Adil Raja Introduction Convex Sets Convex Functions Convex Optimization Problems References INTRODUCTION I I Many situations arise in machine learning where we would like to optimize the value of some function. I That is, given a function f : Rn ! R, we want to find x 2 Rn that minimizes (or maximizes) f (x). I In the general case, finding the global optimum of a function can be a very difficult task. I However, for a special class of optimization problems, known as convex optimization problems, we can efficiently find the global solution in many cases. I “efficiently” has both practical and theoretical connotations: 1. It means that we can solve many real-world problems in a reasonable amount of time.
  • 8. Convex Optimization Muhammad Adil Raja Introduction Convex Sets Convex Functions Convex Optimization Problems References INTRODUCTION II 2. And it means that theoretically we can solve problems in time that depends only polynomially on the problem size. I The goal of these section notes and the accompanying lecture is to give a very brief overview of the field of convex optimization.
  • 9. Convex Optimization Muhammad Adil Raja Introduction Convex Sets Convex Functions Convex Optimization Problems References CONVEX SETS I DEFINITION A set C is convex if, for any x, y 2 C and 2 with 0 1, x + (1 )y 2 C. I Intuitively, this means that if we take any two elements in C, and draw a line segment between these two elements, then every point on that line segment also belongs to C. I Figure 1 shows an example of one convex and one non-convex set. I The point x + (1 )y is called a convex combination of the points x and y.
  • 10. Convex Optimization Muhammad Adil Raja Introduction Convex Sets Convex Functions Convex Optimization Problems References CONVEX SETS II (a) (b) Figure 1: Examples of a convex set (a) and a non-convex set (b). FIGURE: Examples of a convex set (a) and a non-convex set. Examples All of Rn. It should be fairly obvious that given any x, y ! Rn, !x + (1 − !)y ! Rn. The non-negative orthant, Rn Examples I All of Rn. It should be fairly obvious that given any +. The non-negative orthant consists of all vectors in Rn whose elements are all non-negative: Rn + = {x : xi # 0 $i = 1, . . . , n}. To show x; y 2 n; x + (1 )y 2 n. I The non-negative orthant, n that this is a convex set, simply note that given any x, y ! Rn + and 0 % ! % 1, +. The non-negative (!x + (1 − !)y)i = !xi + (1 − !)yi # 0 $i. orthant consists of all vectors in n whose elements are all non-negative: Norm balls. Let · be some norm on Rn (e.g., ! the Euclidean norm, x2 = n x2i ). Then the set {x : x % 1} is a convex set. To see this, suppose x, y ! Rn,
  • 11. Convex Optimization Muhammad Adil Raja Introduction Convex Sets Convex Functions Convex Optimization Problems References CONVEX SETS III 1. n + = x : xi 08i = 1; :::; n. 2. To show that this is a convex set, simply note that given any x; y8Rn+ and 0 ? 1, (x + (1 )y)i = xi + (1 )yi 08i. I Norm balls. Let jj:jj be some norm on n (e.g., the Euclidean norm, x2 = qPni =1 x2 i ). Then the set x : jjxjj 1 is a convex set. I To see this,suppose x; y 2 Rn, with jjxjj 1; jjyjj 1,and 0 1. Then jjx + (1 )yjj jjxjj + jj(1 )yjj = jjxjj + (1 )jjyjj 1 where we used the triangle inequality and the positive homogeneity of norms.
  • 12. Convex Optimization Muhammad Adil Raja Introduction Convex Sets Convex Functions Convex Optimization Problems References CONVEX SETS IV I Affine subspaces and polyhedra. Given a matrix A 2 mn and a vector b 2 m, an affine subspace is the set x 2 n : Ax = b (note that this could possibly be empty if b is not in the range of A). Similarly, a polyhedron is the (again, possibly empty) set x 2 Rn : Ax b, where “” here denotes componentwise inequality (i.e., all the entries of Ax are less than or equal to their corresponding element in b). To prove this, first consider x; y 2 n such that Ax = Ay = b. Then for 0 1, A(x+(1)y) = Ax+(1)Ay = b+(1?)b = b. Similarly,for x; y 2 n that satisfy Ax b andAy b and 0 1, A(x+(1)y) = Ax+(1)Ay b+(1)b = b.
  • 13. Convex Optimization Muhammad Adil Raja Introduction Convex Sets Convex Functions Convex Optimization Problems References CONVEX SETS V I Intersections of convex sets. Suppose CT1,C2,...,Ck are convex sets. Then their intersection ki Ci kix x Cii 1k is also a convex set. =1 = : 2 8= ; :::; TTo see this, consider x; y 2 1 Ci and 0 =? 1. ki Then, x + (1 )y 2 Ci8i = 1; :::; k by the Tdefinition of a convex set. Therefore x + (1 )y 2 =1 Ci . Note, however, that the union of convex sets in general will not be convex. I Positive semidefinite matrices. The set of all symmetric positive semidefinite matrices, often times called the positive semidefinite cone and denoted Sn+ , is a convex set (in general, Sn nn denotes the set of symmetric n ? n matrices). Recall that a matrix A 2 nn is symmetric positive semidefinite if and only if A = AT and for all x 2 Rn, xT Ax 0. Now consider two symmetric positive semidefinite
  • 14. Convex Optimization Muhammad Adil Raja Introduction Convex Sets Convex Functions Convex Optimization Problems References CONVEX SETS VI matrices A;B 2 Sn+ and0 1. Then for any x 2 n, xT (A + (1 )B)x = xT Ax + (1 )xT Bx 0. The same logic can be used to show that the sets of all positive definite, negative definite, and negative semidefinite matrices are each also convex.
  • 15. Convex Optimization Muhammad Adil Raja Introduction Convex Sets Convex Functions Convex Optimization Problems References CONVEX FUNCTIONS I DEFINITION A function f : n ! is convex if its domain (denoted D(f)) is a convex set, and if, for all x; y 2 D(f ) and 2 R; 0 ? 1, f (x + (1 )y) f (x) + (1 )f (y). I Intuitively, the way to think about this definition is that if we pick any two points on the graph of a convex function and draw a straight line between them, then the portion of the function between these two points will lie below this straight line. This situation is pictured in Figure 2. I We say a function is strictly convex if Definition 2 holds with strict inequality for x?6= y and 0 1. We say that f is concave if – f is convex, and likewise that f is strictly concave if – f is strictly convex.
  • 16. Convex Optimization Muhammad Adil Raja Introduction Convex Sets Convex Functions Convex Optimization Problems References CONVEX FUNCTIONS II convex FIGURE: function. Graph of a convex By function. the definition By the definition of of convex functions, the graph must lie above the function. convex functions, the line connecting two points on the graph must lie above the function. Condition for Convexity
  • 17. Convex Optimization Muhammad Adil Raja Introduction Convex Sets Convex Functions Convex Optimization Problems References FIRST ORDER CONDITION FOR CONVEXITY I I Suppose a function f : n ! is differentiable. I The f is convex if and only if D(f ) is a convex set and for all x; y 2 D(f ), f (y) f (x) + 5x f (x)T (y x): I The function 5x f (x)T (y x): is called the first-order approximation to the function f at the point x. I Intuitively, this can be thought of as approximating f with its tangent line at the point x. I The first order condition for convexity says that f is convex if and only if the tangent line is a global underestimator of the function f . I In other words, if we take our function and draw a tangent line at any point, then every point on this line will lie below the corresponding point on f . I A function is differentiable if the gradient 5f (x) exists at all points x in the domain of f .
  • 18. Convex Optimization Muhammad Adil Raja Introduction Convex Sets Convex Functions Convex Optimization Problems References FIRST ORDER CONDITION FOR CONVEXITY II I The gradient is defined as 5x f (x) 2 n; (5x f (x))i = @f (x) @(x)i . I Similar to the definition of convexity, f will be strictly convex if this holds with strict inequality, concave if the inequality is reversed, and strictly concave if the reverse inequality is strict.
  • 19. corresponding point on f. the definition of convexity, f will be strictly convex if this holds Convex with Optimization concave if the inequality is reversed, and strictly concave if the Muhammad reverse Adil inequality Raja Introduction Convex Sets Convex Functions Convex Optimization Problems References FIRST ORDER CONDITION FOR CONVEXITY III Figure 3: Illustration of first-order condition for convexity. the gradient is defined as xf(x) # Rn, (xf(x))i = !f(x) . For a review on gradients !xi previous section notes on linear algebra. FIGURE: Illustration od the first-order condition for convexity.
  • 20. Convex Optimization Muhammad Adil Raja Introduction Convex Sets Convex Functions Convex Optimization Problems References SECOND ORDER CONDITION FOR CONVEXITY I 2x I Suppose a function f : n ! is twice differentiable. I Then f is convex if and only if D(f ) is a convex set and its Hessian is positive semidefinite: i.e., for any x 2 D(f ), 5f (x) 0. I Here, the notation “” when used in conjunction with matrices refers to positive semidefiniteness, rather than componentwise inequality. I In one dimension, this is equivalent to the condition that the second derivative f 00(x) always be positive (i.e., the function always has positive curvature).
  • 21. Convex Optimization Muhammad Adil Raja Introduction Convex Sets Convex Functions Convex Optimization Problems References SECOND ORDER CONDITION FOR CONVEXITY II 2x I Again analogous to both the definition and first order conditions for convexity, f is strictly convex if its Hessian is positive definite, concave if the Hessian is negative semidefinite, and strictly concave if the Hessian is negative definite. I A function is twice differentiable if the Hessian 5f (x) is defined for all points x in the domain of f . 2x I The Hessian is defined as 5f (x) 2 nn; 2x (5f (x))ij = @2f (x) @xi@xj . I For a symmetric matrix X 2 Sn;X 0 denotes that X is negative semidefinite. I As with inequalities, “” and “”.
  • 22. Convex Optimization Muhammad Adil Raja Introduction Convex Sets Convex Functions Convex Optimization Problems References JENSEN’S INEQUALITY I I Suppose we start with the inequality in the basic definition of a convex function f (x + (1 )y) f (x) + (1 )f (y)for0 1. I Using induction, this can be fairly easily extended to convex combinations of more than one point, f ( Pki =1 ixi ) Pki =1 i f (xi ) for Pki =1 i = 1; i 08i. I In fact, this can also be extended to infinite sums or integrals. I In the latter case, the inequality can be written as f ( R p(x)xdx) R R p(x)f (x)dx for p(x)dx = 1; p(x) 08x. I Because p(x) integrates to 1, it is common to consider it as a probability density, in which case the previous equation can be written in terms of expectations, f (E[x]) E[f (x)]. I This last inequality is known as JensenÕs inequality.
  • 23. Convex Optimization Muhammad Adil Raja Introduction Convex Sets Convex Functions Convex Optimization Problems References SUBLEVEL SETS I I Convex functions give rise to a particularly important type of convex set called an -sublevel set. I Given a convex function f : n ! and a real number 2 , the -sublevel set is defined as x 2 D(f ) : f (x) . I In other words, the -sublevel set is the set of all points x such that f (x) . I To show that this is a convex set, consider any x; y 2 D(f ) such that f (x) and f (y) . I Then f (x + (1 )y) f (x) + (1 )f (y) + (1 ) = .
  • 24. Convex Optimization Muhammad Adil Raja Introduction Convex Sets Convex Functions Convex Optimization Problems References EXAMPLES OF CONVEX FUNCTIONS I I We begin with a few simple examples of convex functions of one variable, then move on to multivariate functions. I Exponential. Let f : ! ; f (x) = eax for any a 2 . To show f is convex, we can simply take the second derivative f 00(x) = a2eax , which is positive for all x. I Negative logarithm. Let f : ! ; f (x) =?logx with domain D(f ) = R++. I Here, R++ denotes the set of strictly positive real numbers, x : x 0. I Then f 00(x) = 1=x2 0 for all x. I Affine functions. Let f : n ! ; f (x) = bT x + c for some b 2 n, c 2 . I In this case the Hessian, 52xf (x) = 0 for all x.
  • 25. Convex Optimization Muhammad Adil Raja Introduction Convex Sets Convex Functions Convex Optimization Problems References EXAMPLES OF CONVEX FUNCTIONS II 12 I Because the zero matrix is both positive semidefinite and negative semidefinite, f is both convex and concave. I In fact, affine functions of this form are the only functions that are both convex and concave. I Quadratic functions. Let f : n ! ; f (x) = xT Ax + bT x + c for a symmetric matrix A 2 Sn, b 2 n and c 2 . I The Hessian for this function is given by 52xf (x) = A. I Therefore, the convexity or non-convexity of f is determined entirely by whether or not A is positive semidefinite: if A is positive semidefinite then the function is convex (and analogously for strictly convex, concave, strictly concave).
  • 26. Convex Optimization Muhammad Adil Raja Introduction Convex Sets Convex Functions Convex Optimization Problems References EXAMPLES OF CONVEX FUNCTIONS III 22 I If A is indefinite then f is neither convex nor concave. I Note that the squared Euclidean norm f (x) = jjxjj= xT x is a special case of quadratic ni functions where A = I; b = 0; c = 0, so it is therefore a strictly convex function. I Norms. Let f ;n ! be some norm on n. Then by the triangle inequality and positive homogeniety of norms, for x; y 2 n; 0 1, f (x + (1 )y) f (x) + f (1 )y) = f (x) + (1 )f (y). I This is an example of a convex function where it is not possible to prove convexity based on the second or first order conditions. I The reason is that because norms are not generally differentiable Peverywhere (e.g., the 1-norm, jjxjj1 = =1 jxi j, is non-differentiable at all points where any xi is equal to zero).
  • 27. Convex Optimization Muhammad Adil Raja Introduction Convex Sets Convex Functions Convex Optimization Problems References EXAMPLES OF CONVEX FUNCTIONS IV I Nonnegative weighted sums of convex functions. Let f1; f2; :::; fk be convex functions and w1;w2; :::;wk be nonnegative real numbers. Then f (x) = Pki =1 wi fi (x) is a convex function, since f (x + (1?)y) = Pki =1 wi fi (x + (1 )y) (1) Pki =1 wi (fi (x) + (1 )fi (y)) (2) = Pki =1 wi fi (x) + (1 ) Pki =1 wi fi f (y)(3) = f (x) + (1 )f (x) (4)
  • 28. Convex Optimization Muhammad Adil Raja Introduction Convex Sets Convex Functions Convex Optimization Problems References CONVEX OPTIMIZATION PROBLEMS I I Armed with the definitions of convex functions and sets, we are now equipped to consider convex optimization problems. I Formally, a convex optimization problem in an optimization problem of the form minimize f (x) subject to x 2 C. I where f is a convex function, C is a convex set, and x is the optimization variable. I However, since this can be a little bit vague, we often write it often written as minimize f (x) subject to gi (x) 0, i = 1; :::;m hi (x) = 0; i = 1; :::; p I where f is a convex function, gi are convex functions, and hi are affine functions, and x is the optimization variable.
  • 29. Convex Optimization Muhammad Adil Raja Introduction Convex Sets Convex Functions Convex Optimization Problems References CONVEX OPTIMIZATION PROBLEMS II I It is important to note the direction of these inequalities: 1. A convex function gi must be less than zero. 2. This is because the 0-sublevel set of gi is a convex set, so the feasible region, which is the intersection of many convex sets, is also convex (recall that affine subspaces are convex sets as well). 3. If we were to require that gi 0 for some convex gi , the feasible region would no longer be a convex set, and the algorithms we apply for solving these problems would not longer be guaranteed to find the global optimum. 4. Also notice that only affine functions are allowed to be equality constraints. 5. Intuitively, you can think of this as being due to the fact that an equality constraint is equivalent to the two inequalities hi 0 and hi 0.
  • 30. Convex Optimization Muhammad Adil Raja Introduction Convex Sets Convex Functions Convex Optimization Problems References CONVEX OPTIMIZATION PROBLEMS III 6. However, these will both be valid constraints if and only if hi is both convex and concave, i.e., hi must be affine. I The optimal value of an optimization problem is denoted p (or sometimes f ) and is equal to the minimum possible value of the objective function in the feasible region. p = minf (x) : gi (x) 0; i = 1; :::;m; hi (x) = 0; i = 1; :::; p. I We allow p to take on the values +1 and 1 when the problem is either infeasible (the feasible region is empty) or unbounded below (there exists feasible points such that f (x) ! 1), respectively. I We say that x is an optimal point if f (x?) = p. I Note that there can be more than one optimal point, even when the optimal value is finite.
  • 31. Convex Optimization Muhammad Adil Raja Introduction Convex Sets Convex Functions Convex Optimization Problems References GLOBAL OPTIMALITY IN CONVEX PROBLEMS I I Defining the concepts of local optima and global optima are in order. I Intuitively, a feasible point is called locally optimal if there are no “nearby” feasible points that have a lower objective value. I Similarly, a feasible point is called globally optimal if there are no feasible points at all that have a lower objective value. I To formalize this a little bit more, following definitions are important.
  • 32. Convex Optimization Muhammad Adil Raja Introduction Convex Sets Convex Functions Convex Optimization Problems References GLOBAL OPTIMALITY IN CONVEX PROBLEMS II DEFINITION A point x is locally optimal if it is feasible (i.e., it satisfies the constraints of the optimization problem) and if there exists some R 0 such that all feasible points z with jjx zjj2 R, satisfy f (x) f (z). DEFINITION A point x is globally optimal if it is feasible and for all feasible points z, f (x) f (z). I We now come to the crucial element of convex optimization problems, from which they derive most of their utility.
  • 33. Convex Optimization Muhammad Adil Raja Introduction Convex Sets Convex Functions Convex Optimization Problems References GLOBAL OPTIMALITY IN CONVEX PROBLEMS III I The key idea is that for a convex optimization problem all locally optimal points are globally optimal. I LetÕs give a quick proof of this property by contradiction. I Suppose that x is a locally optimal point which is not globally optimal, i.e., there exists a feasible point y such that f (x) f (y). I By the definition of local optimality, there exist no feasible points z such that jjx?zjj2 Randf (z) f (x).
  • 34. Convex Optimization Muhammad Adil Raja Introduction Convex Sets Convex Functions Convex Optimization Problems References GLOBAL OPTIMALITY IN CONVEX PROBLEMS IV I But now suppose we choose the point z = y + (1 )x with = R2 jjx?yjj2 Then jjx zjj2 = jjx ( R 2jjxyjj2 y + (1 R 2jjxyjj2)x)jj2 (5) = jj R 2jjxyjj2 (x y)jj2 (6) = R=2 R: (7) I In addition, by the convexity of f we have f (z) = f (y + (1?)x) f (y) + (1?)f (x) f (x). I Furthermore, since the feasible set is a convex set, and since x and y are both feasible z = y + (1?) will be feasible as well. I Therefore, z is a feasible point, with jx?zj2 R and f (z) f (x).
  • 35. Convex Optimization Muhammad Adil Raja Introduction Convex Sets Convex Functions Convex Optimization Problems References GLOBAL OPTIMALITY IN CONVEX PROBLEMS V I This contradicts our assumption, showing that x cannot be locally optimal.
  • 36. Convex Optimization Muhammad Adil Raja Introduction Convex Sets Convex Functions Convex Optimization Problems References SPECIAL CASES OF CONVEX PROBLEMS I I For a variety of reasons, it is often times convenient to consider special cases of the general convex programming formulation. I For these special cases we can often devise extremely efficient algorithms that can solve very large problems, and because of this you will probably see these special cases referred to any time people use convex optimization techniques. I Linear Programming. We say that a convex optimization problem is a linear program (LP) if both the objective function f and inequality constraints gi are affine functions. I In other words, these problems have the form minimize cT x + d subject toGx h.
  • 37. Convex Optimization Muhammad Adil Raja Introduction Convex Sets Convex Functions Convex Optimization Problems References SPECIAL CASES OF CONVEX PROBLEMS II I Ax = b where x 2 n is the optimization variable, c 2 n, d 2 , G 2 mn, h 2 m, A 2 p?n, b 2 Rp are defined by the problem, and “” denotes elementwise inequality. I Quadratic Programming. We say that a convex optimization problem is a quadratic program (QP) if the inequality constraints gi are still all affine, but if the objective function f is a convex quadratic function. I In other words, these problems have the form, minimize 1 2xT Px + cT x + d subject to Gx h + Ax = nb. where again x 2 n is the optimization variable, c 2 n, d 2 , G 2 mn, h 2 Rm, A 2 pn, b 2 Rp are defined by the problem, but we also have P 2 S, a symmetric positive semidefinite matrix.
  • 38. Convex Optimization Muhammad Adil Raja Introduction Convex Sets Convex Functions Convex Optimization Problems References SPECIAL CASES OF CONVEX PROBLEMS III I Quadratically Constrained Quadratic Programming. We say that a convex optimization problem is a quadratically constrained quadratic program (QCQP) if both the objective f and the inequality constraints gi are convex quadratic functions, minimize 1 2xT Px + cT x + d subject to 1 2xTQix + r T i x + si 0; i = 1; :::;m Ax = b where, as before, x 2 n is the optimization variable, c 2 n, d 2 , A 2 pn, b 2 p, P 2 Sn+ , but we also have Qi 2 Sn+ ; ri 2 n; si 2 ,for i = 1,...,m. I Semidefinite Programming has become more and more prevalent in many different areas of machine learning research, so you might encounter these at some point, and it is good to have an idea of what they are.
  • 39. Convex Optimization Muhammad Adil Raja Introduction Convex Sets Convex Functions Convex Optimization Problems References SPECIAL CASES OF CONVEX PROBLEMS IV I We say that a convex optimization problem is a semidefinite program (SDP) if it is of the form minimizetr (CX) subject to tr (AiX) = bi , i = 1; :::; p X 0. I where the symmetric matrix X 2 Sn is the optimization variable, the symmetric matrices C, A1; :::; Ap 2 Sn are defined by the problem, and the constraint X 0 means that we are constraining X to be positive semidefinite. I This looks a bit different than the problems we have seen previously, since the optimization variable is now a matrix instead of a vector.
  • 40. Convex Optimization Muhammad Adil Raja Introduction Convex Sets Convex Functions Convex Optimization Problems References SPECIAL CASES OF CONVEX PROBLEMS V I It should be fairly obvious from the definitions that quadratic programs are more general than linear programs (since a linear program is just a special case of a quadratic program where P = 0), and likewise that quadratically constrained quadratic programs are more general than quadratic programs. I However, what is not obvious at all is that semidefinite programs are in fact more general than all the previous types. I That is, any quadratically constrained quadratic program (and hence any quadratic program or linear program) can be expressed as a semidefinte program. I
  • 41. Convex Optimization Muhammad Adil Raja Introduction Convex Sets Convex Functions Convex Optimization Problems References EXAMPLES OF CONVEX OPTIMIZATION I I Support Vector Machines. I Constrained Least Squares. I Maximum likelihood for Logistic Regression.
  • 42. Convex Optimization Muhammad Adil Raja Introduction Convex Sets Convex Functions Convex Optimization Problems References REFERENCES I I The images and inspiration for preparing these slides have been taken from Andrew Ngs course on Machine Learning. I The course is available on Stanford engineering for all and Coursera.