3. The Three Problems of
Polyhedral Computation
● Representation Conversion
● H-representation of polyhedron
● V-Representation of polyhedron
● Projection
●
● Redundancy Removal
● Compute the minimal representation of a polyhedron
October 3, 2012 Jayant Apte. ASPITRG 3
4. Outline
● Preliminaries
● Representations of convex polyhedra, polyhedral cones
● Homogenization - Converting a general polyhedron in to
a cone in
● Polar of a convex cone
● The three problems
● Representation Conversion
– Review of LRS
– Double Description Method
● Projection of polyhedral sets
– Fourier-Motzkin Elimination
– Block Elimination
– Convex Hull Method (CHM)
● Redundancy removal
– Redundancy removal using linear programming
October 3, 2012 Jayant Apte. ASPITRG 4
5. Outline for Part 1
● Preliminaries
● Representations of convex polyhedra, polyhedral cones
● Homogenization – Converting a general polyhedron in
to a cone in
●
Polar of a convex cone
● The first problem
● Representation Conversion
– Review of LRS
– Double Description Method
October 3, 2012 Jayant Apte. ASPITRG 5
11. Example
(0.5,0.5,1.5)
(1,1,1)
(0,1,1)
(0,0,1) (1,1,0)
(0,1,0)
(1,0,0)
(0,0,0)
October 3, 2012 Jayant Apte. ASPITRG 11
12. Switching between the two representations :
Representation Conversion Problem
● H-representation V-representation: The
Vertex Enumeration problem
● Methods:
● Reverse Search, Lexicographic Reverse Search
● Double-description method
● V-representation H-representation
The Facet Enumeration Problem
● Facet enumeration can be accomplished by using
polarity and doing Vertex Enumeration
October 3, 2012 Jayant Apte. ASPITRG 12
26. Use of Polar
● Representation Conversion
● No need to have two different algorithms i.e. for
and .
● Redundancy removal
● No need to have two different algorithms for
removing redundancies from H-representation
and V-representation
● Safely assume that input is always an H-
representation for both these problems
October 3, 2012 Jayant Apte. ASPITRG 26
31. Reverse Search:
High Level Idea
● Start with dictionary corresponding to the optimal
vertex
● Ask yourself 'What pivot would have landed me at this
dictionary if i was running simplex?'
● Go to that dictionary by applying reverse pivot to
current dictionary
● Ask the same question again
● Generate the so-called 'reverse search tree'
October 3, 2012 Jayant Apte. ASPITRG 31
32. Reverse Search on a Cube
Ref.David Avis, lrs: A Revised Implementation of the Reverse Search Vertex Enumeration
Algorithm
October 3, 2012 Jayant Apte. ASPITRG 32
33. Double Description Method
● First introduced in:
“ Motzkin, T. S.; Raiffa, H.; Thompson, G. L.; Thrall, R. M.
(1953). "The double description method". Contributions to the
theory of games. Annals of Mathematics Studies. Princeton, N.
J.: Princeton University Press. pp. 51–73”
● The primitive algorithm in this paper is very inefficient.
(How? We will see later)
● Several authors came up with their own efficient
implementation(viz. Fukuda, Padberg)
● Fukuda's implementation is called cdd
October 3, 2012 Jayant Apte. ASPITRG 33
35. Double Description Method:
The High Level Idea
● An Incremental Algorithm
● Starts with certain subset of rows of H-representation
of a cone to form initial H-representation
● Adds rest of the inequalities one by one constructing
the corresponding V-representation every iteration
● Thus, constructing the V-representation incrementally.
October 3, 2012 Jayant Apte. ASPITRG 35
60. Efficiency Issues
● Is the implementation I just described good
enough?
October 3, 2012 Jayant Apte. ASPITRG 60
61. Efficiency Issues
● Is the implementation I just described good
enough?
● Hell no!
– The implementation just described suffers from
profusion of redundancy
October 3, 2012 Jayant Apte. ASPITRG 61
62. Efficiency Issues We can
totally
do
without
this one!!
October 3, 2012 Jayant Apte. ASPITRG 62
63. Efficiency Issues And without
all these!!!
October 3, 2012 Jayant Apte. ASPITRG 63
82. How it works? Contd...
October 3, 2012 Jayant Apte. ASPITRG 82
83. Efficiency of FM algorithm
October 3, 2012 Jayant Apte. ASPITRG 83
84. Convex Hull Method(CHM)
● First appears in “C. Lassez and J.-L. Lassez, Quantifier
elimination for conjunctions of linear constraints via
aconvex hull algorithm, IBM Research Report, T.J. Watson
Research Center (1991)”
● Found to be better than most other existing algorithms
when dimension of projection is small
● Cited by Weidong Xu, Jia Wang, Jun Sun in their ISIT
2008 paper “A Projection Method for Derivation of Non-
Shannon-Type Information Inequalities”
October 3, 2012 Jayant Apte. ASPITRG 84
89. Moral of the story
● We can make decisions about without
actually having its H-representation.
● It suffices to have , the original polyhedron
● We run linear programs on to make these
decisions
October 3, 2012 Jayant Apte. ASPITRG 89
90. Input:
Faces A of P
Construct an initial
Approximation of
extreme points of
Perform Facet Update by adding r to it
Enumeration on
to get
Convex Hull Method:
Flowchart
Take an inequality from
that is not a face of
and move it outward
To find a new extreme point r
Are all inequalities
In faces
No
Of
?
Yes
Stop
October 3, 2012 Jayant Apte. ASPITRG 90
94. Input:
Faces A of P
Construct an initial
Approximation of
extreme points of
Perform Facet Update by adding r to it
Enumeration on
Convex Hull Method: to get
Flowchart
Take an inequality from
that is not a face of
and move it outward
To find a new extreme point r
Are all inequalities
In facets
Of
?
Stop
October 3, 2012 Jayant Apte. ASPITRG 94
95. How to get the extreme points of
initial approximation?
October 3, 2012 Jayant Apte. ASPITRG 95
96. Example: Get the first two points
October 3, 2012 Jayant Apte. ASPITRG 96
97. Example: Get the first two points
October 3, 2012 Jayant Apte. ASPITRG 97
98. Example: Get the first two points
October 3, 2012 Jayant Apte. ASPITRG 98
99. Get rest of the points so you have a
total of d+1 points
October 3, 2012 Jayant Apte. ASPITRG 99
100. Get the rest of the points so you
have a total of d+1 points
October 3, 2012 Jayant Apte. ASPITRG 100
101. Get the rest of the points so you
have a total d+1 points
October 3, 2012 Jayant Apte. ASPITRG 101
102. How to get the initial facets of the
projection?
October 3, 2012 Jayant Apte. ASPITRG 102
103. How to get the initial facets of the
projection?
October 3, 2012 Jayant Apte. ASPITRG 103
104. Input:
Faces A of P
Construct an initial
Approximation of
extreme points of
Incrementally update Update by adding r to it
The Convex Hull
Convex Hull Method: (Perform Facet
Enumeration on
Flowchart to get )
Take an inequality from
that is not a face of
and move it outward
To find a new extreme point r
Are all inequalities
In faces No
Of
?
Yes
Stop
October 3, 2012 Jayant Apte. ASPITRG 104
105. Input:
Faces A of P
Construct an initial
Approximation of
extreme points of
Incrementally update Update by adding r to it
The Convex Hull
Convex Hull Method: (Perform Facet
Enumeration on
Flowchart to get )
Take an inequality from
that is not a face of
and move it outward
To find a new extreme point r
Are all inequalities
In faces No
Of
?
Yes
Stop
October 3, 2012 Jayant Apte. ASPITRG 105
106. Input:
Faces A of P
Construct an initial
Approximation of
extreme points of
Incrementally update Update by adding r to it
The Convex Hull
Convex Hull Method: (Perform Facet
Enumeration on
Flowchart to get )
Take an inequality from
that is not a facet of
and move it outward
To find a new extreme point r
Are all inequalities
In faces No
Of
?
Yes
Stop
October 3, 2012 Jayant Apte. ASPITRG 106
107. Incremental refinement
● Find a facet in current approximation of
that is not actually the facet of
● How to do that?
October 3, 2012 Jayant Apte. ASPITRG 107
108. Incremental refinement
This inequality is
Not a facet of
October 3, 2012 Jayant Apte. ASPITRG 108
110. Input:
Faces A of P
Construct an initial
Approximation of
extreme points of
Incrementally update Update by adding r to it
The Convex Hull
Convex Hull Method: (Perform Facet
Enumeration on
Flowchart to get )
Take an inequality from
that is not a facet of
and move it outward
To find a new extreme point r
Are all inequalities
No
In faces
Of
?
Yes
Stop
October 3, 2012 Jayant Apte. ASPITRG 110
111. How to update the H-representation
to accommodate new point ?
October 3, 2012 Jayant Apte. ASPITRG 111
115. How to update the H-representation
to accommodate new point ?
Determine validity of
The candidate facet
And also its sign
October 3, 2012 Jayant Apte. ASPITRG 115
125. Comparison of Projection
Algorithms
● Fourier Motzkin Elimination and Block
Elimination doesn't work well when used on big
problems
● CHM works very well when the dimension of
projection is relatively small as compared to the
dimention of original polyhedron.
● Weidong Xu, Jia Wang, Jun Sun have already
used CHM to get non-Shannon inequalities.
October 3, 2012 Jayant Apte. ASPITRG 125
126. Computation of minimal
representation/Redundancy
Removal
October 3, 2012 Jayant Apte. ASPITRG 126
128. References
●
K. Fukuda and A. Prodon. Double description method revisited. Technical report, Department of Mathematics,
Swiss Federal Institute of Technology, Lausanne, Switzerland, 1995
●
G. Ziegler, Lectures on Polytopes, Springer, 1994, revised 1998. Graduate Texts in Mathematics.
●
Tien Huynh, Catherine Lassez, and Jean-Louis Lassez. Practical issues on the projection of polyhedral sets.
Annals of mathematics and artificial intelligence, November 1992
●
Catherine Lassez and Jean-Louis Lassez. Quantifier elimination for conjunctions of linear constraints via a
convex hull algorithm. In Bruce Donald, Deepak Kapur, and Joseph Mundy, editors, Symbolic and Numerical
Computation for Artificial Intelligence. Academic Press, 1992.
●
R. T. Rockafellar. Convex Analysis (Princeton Mathematical Series). Princeton Univ Pr.
●
W. Xu, J. Wang, J. Sun. A projection method for derivation of non-Shannon-type information inequalities. In
IEEE International Symposiumon Information Theory (ISIT), pp. 2116 – 2120, 2008.
October 3, 2012 Jayant Apte. ASPITRG 128
129. Minkowski's Theorem for
Polyhedral Cones
Weyl's Theorem for
Polyhedral Cones
October 3, 2012 Jayant Apte. ASPITRG 129