# User's Guide for lrs - Version 3.2a

### David Avis          avis@cs.mcgill.ca       http://www.cs.mcgill.ca/~avis

• Introduction
• Installation
• File Formats
• Basic Options
• Estimation
• Linear Programming
• Volume Computation
• Voronoi Diagrams
• Extreme Point Enumeration and Redundant Inequalities
• Timing and Interrupts
• Error Messages and Troubleshooting
• Acknowledgements and References
• ### Introduction

A polyhedron can be described by a list of inequalities (H-representation) or as by a list of its vertices and extreme rays (V-representation). lrs is a C program that converts a H-representation of a polyhedron to its V-representation, and vice versa.  These problems are known respectively at the vertex enumeration and convex hull problems. lrs is based on the reverse search algorithm of Avis and Fukuda(1992), modified to use lexicographic pivoting  and implemented in rational arithmetic. See Avis(1998a) for a technical description, and Avis(1998b) for some computational experience. The input files are in Polyhedra format, developed by Fukuda and the author. The format is essentially self-dual, and the output file produced can be read in as an input file, with very minor modifications, to perform the reverse transformation. This format is compatible with that  used in Fukuda's cdd package, which performs the same transformations using a version of the double description methodcdd can also be used  in conjunction with lrs as a pre-processor for projections to subspaces, or as a post processor for computing the entire face lattice. Another program using the same file format is the primal-dual method pd, developed by Bremner, Fukuda and Marzetta(1997).  It is essentially dual to lrs, and is very efficient for computing H-representations of simple polyhedra, and V-representations of simplicial polyhedra. It will compute the volume of a polytope given by an H-representation. The cdd manual contains a more detailed introduction to the problem, along with many useful tips for the new user. Another program for based on the double description method is Christof and Loebel's portaand a versatile tool for the algorithmic treatment of polytopes is Gawrilow and Joswig's  polymake package. Barber et al.'s qhull  and some visualization programs, can be found at the Geometry Center. A package for volume computation has been developped by Bueeler and  Enge.  A comprehensive general source of related infomation are Erickson's Computational Geometry Pages.

Polyhedra handled by lrs must be full dimensional and contain at least one vertex.  lrs accepts either integer or rational input, and produces integer or rational output. All computations are done exactly using extended precision arithmetic. Since it is a pivot based method, it can be very slow for degenerate inputs: i.e.. H-representations of non-simple polyhedra, and V-representations of non-simplicial polyhedra. On the other hand, it does not store the vertices/ rays or facets produced, so for very large problems it may be the only method that can solve the problem.  A discussion of various vertex enumeration/convex hull methods and the types of polyhedra that cause them to behave badly is contained in Avis, Bremner and Seidel( 1997).

• Estimating the number of vertices/rays or facets of a polytope, and estimating the total running time
• Computing the volume of a polytope given by a V-representation
• Solving LP problems for a polyhedron given by a H-representation
• Computing the Voronoi vertices and rays for an input set of data points
• The ability to suspend and restart execution at any time

This program can be distributed freely under the GNU GENERAL PUBLIC LICENSE. Please read the file COPYING carefully before using.  Please inform the author of any interesting applications for which lrs was helpful.

### Installation

• Access the ftp site (ftp://mutt.cs.mcgill.ca/pub/C)   and copy  at least the files lrs3.2.c, buffer.c, cube.ine, cube.ext
• Compile using the command
• gcc -O -o lrs lrs3.2a.c

or if you have a 64 bit machine (e.g.. DEC Alpha)

gcc -O -DB64 -o lrs lrs3.2a.c

If you have trouble compiling, try compile option
-DOMIT_SIGNALS  and/or -DOMIT_TIMES

•  Compile buffer.c using the command
• gcc -O -o buffer buffer.c

• Test the program by typing:

lrs < cube.ine

This will convert the H-representation of a cube given cube by 6 the six inequalities -1 <= xi <= 1 , i=1,2,3 given by the input:
cube
*cube of side 2 centred at the origin
H-representation
begin
6 4 rational
1 1 0 0
1 0 1 0
1 0 0 1
1 -1 0 0
1 0 -1 0
1 0 0 -1
end

The output should contain (along with other comment lines beginning with a "*")

cube
V-representation
begin
***** 4 rational
1 1 1 1
1 -1 1 1
1 1 -1 1
1 -1 -1 1
1 1 1 -1
1 -1 1 -1
1 1 -1 -1
1 -1 -1 -1
end

This is a list of the 8 vertices with each co-ordinate +/- 1.  The ***** should be replaced by the actual number, 8, of vertices. Since lrs does not save the output produced, it does not know this value until the execution terminates. This output is now essentially the same as file cube.ext. To complete the test type:

lrs < cube.ext

Now the output produced is essentially the file cube.ine, with the inequalities appearing in a different order.

The program buffer is optional. It is used to remove duplicate line in the output they may occur if the input is not a polytope or a pointed cone. For such unbounded polyhedra,  rays may appear in the output more than once. The program takes two optional integer parameters:

buffer  maxline maxbuffer

It builds a circular buffer of maxbuffer (default 50) lines of maxline (default 5000)characters each. If an output line is already in the buffer, it is not output.  To use buffer, pipe the output from lrs:

lrs < filename | buffer

### File Formats

The input for lrs is a H- or V- representation of a polytope. These files have the following formats:

#### H-representation

name
H-representation
begin
m n rational

{list of inequalities }

end
{options)

name is a user supplied name for the polytope.  If the line H-representation is omitted, H-representation is assumed.  The input coefficients are read in free format, and are not checked for type. Coefficients are separated by white space. Normally this file would be saved with filename suffix .ine but this not required.  Comments may appear before the begin or after the end, and to avoid interpretation as an option, should begin with a special character such as "*" or "#".
The integer  m is the number of inequalities,  and the integer n is the dimension of the input +1.
A list of inequalities contains the coefficients of inequalities of the form

a0 + a1 x1 + ... + an-1 xn-1 >=  0.

This inequality is input as the line

a0 a1 ... an-1

The coefficients can be entered as integers or rationals in the format x/y.
For example, the square centred at the origin with side length two has inequalities

1 + x1 >= 0      1+ x2 >=0         1-x1 >= 0           1-x2 >=0

and would be represented by the input file

square
*centred square of side 2
H-representation
begin
4 3 rational
1 1 0
1 0 1
1 -1 0
1 0 -1
end

#### V-representation

name
V-representation
begin
m n rational

{list of vertices and extreme rays}
end
{options)

name is a user supplied name for the polytope.  The line V-representation is required.  The input coefficients are read in free format, and are not checked for type. Coefficients are separated by white space. Comments may appear before the begin or after the end, and to avoid interpretation as an option, should begin with a special character such as "*" or "#".Normally this file would be stored with filename suffix .ext, but this is not required.

The integer  m is the number of vertices and rays,  and the integer n is the dimension of the input +1.
Each vertex is given in the form

1   v0   v 1 ...   vn-1

Each ray is given in the form

0   r0   r 1 ...   rn-1

where r0   r 1 ...   rn-1is a point on the ray.

There must be at least one vertex in each file. For bounded polyhedra there will be no rays entered.
The coefficients can be entered as integers or rationals in the format x/y.
For example, the unit square centred at the origin has vertices
(1,1) ,(1,-1),(-1,1),(-1,-1)
and would be represented by the input file

square
*centred square of side 2
V-representation
begin
4 3 rational
1 1 1
1 1 -1
1 -1 1
1 -1 -1
end

The positive quadrant has vertex (0,0) and rays (1,0) (0,1) and is represented

V-representation
begin
3 3 rational
1 0 0
0 1 0
0 0 1
end

Its H-representation contains the inequalities x1 >= 0 and x2 >= 0 :

H-representation
begin
2 3 rational
0 1 0
0 0 1
end

Note for cdd users: lrs uses essentially the same file format as cdd. Files prepared for cdd should work with little or no modification. Note that  the V-representation corresponds to the "hull" option in cdd. Options specific to cdd can be left in the input files and will be ignored by lrs.  Note the input files for lrs are read in free format, after the line m n rational lrs will look for exactly m*n rationals or integers separated by white space (blank,  carriage return, tab etc.). lrs will not "drop" extra columns of input if n is less than the number of columns supplied.

### Basic Options

lrs has many options to allow various functions to be performed, and to modify the output produced. Almost all options are placed after the end statement, maintaining compatibility with cdd. Where this is not the case, it will be mentioned explicitly. All options are optional.

allbases

This option instructs lrs to list each vertex (or facet) for each of its bases. Normally a vertex (or facet) is only output when its lex-min basis is found, to avoid duplications - see section Output Duplication . This option is often combined with printcobasis.

cache n

lrs stores the latest  n dictionaries in the reverse search tree. This speeds up the backtracking step, but requires more memory.  If n is set to one, there is no caching and "pure" reverse search is performed.  The default is n=10. At the end of a run a message gives cache information. The output
*Dictionary Cache: max size= 4 misses= 10/1340      Tree Depth=5
indicates that with cache size set to 4, only 10  of the 1340 dictionaries computed were not in the cache  during backtracking and had to be recomputed. The maximum tree depth was 5, so there would be no misses with a cache size of 6. Full caching reduces processing time by about 40%.

digits n                // placed before the begin statement//

n is the maximum number of decimal digits to be used. If this is exceeded the program terminates with a message (it can  usually be restarted).   The default is set to about 100 digits. At the end of a run a message is given informing the user of the maximum integer size encountered. This may be used to optimize memory usage and speed on subsequent runs (if doing estimation for example). The output:
*Max digits= 45/100
indicates that the maximum integer encountered had 45 decimal digits, and the program allowed up to 100 digit integers.

estimates k

Estimate the output size. Used in conjunction with maxdepth - see Estimation.

geometric                 // H-representation  or voronoi option only //

With this option, each ray is printed together with the vertex with which it is incident. They output has the form:

0   r0   r 1 ...   rn-1* 1   v0   v 1 ...   vn-1

This indicates ray  0   r0   r 1 ...   rn-1is incident with vertex  1   v0   v 1 ...   vn-1.  For the quadrant, the output is:

1 0 0
0 0 1 * 1 0 0
0 1 0 * 1 0 0
This indicates the origin is a vertex, and there are two geometric rays: (0,t) and (t,0) both adjacent to the origin. For more information see Geometric Rays in Hints and Comments.

maxdepth k

The search will be truncated at depth k. All bases with depth less than or equal to k will be computed.  k is  a non-negative integer, and this option is used for estimates - see Estimation.
Note: For H-representations, rays at depth k will not be reported. For V-representations, facets at depth k will not be reported.

maximize                                              // H-representation  only //
minimize

mindepth k

Backtracking will be terminated at depth k, for k a non-negative integer. This can be used for running reverse search on subtrees as separate processes, e.g. in a distributed computing environment.

printcobasis  k

Every k'th cobasis is printed. If k is omitted, k=1 is assumed and all cobases are printed. For a long run it is useful to print the cobasis occasionally so that the program can be restarted if necessary.
H-representation:  If the input is an H-representation the cobasis is a list the indices of the inequalities from the input file that define the current vertex or ray. For example, with input cube.ine a typical output is:
V#5 R#0 B#5 h=1 facets  3 4 5 det=1
1 1 1 -1
This indicates that the vertex (1,1,-1) is defined by the 3rd, 4th and 5th facet inequalities in the input file being satisfied as equations. It is the B#=5th basis computed, and there have been V#=5 vertices and R#=0 rays output up to this point.
For rays, a cobasis is also printed. In this case the cobasis is the cobasis of the vertex from which the ray emanates. One of the indices is starred, this indicates the inequality to be dropped from the cobasis to define the ray. For example the output:
V#1 R#6 B#4 h=1 facets  2 4* 5 7 det=1
0 1 1 2 1
indicates that the ray (1,1,2,1) emanates from a vertex with cobasis defined by input inequalities 2 4 5 7 satisfied as equations. The ray is defined by dropping the equation with index 4. Note that there may not appear any vertex in the output with cobasis 2 4 5 7, since the corresponding vertex may be degenerate and printed with another (the lex-min) cobasis. To find out which vertices correspond to which rays, use also the geometric option.  Now the output may appear:
V#1 R#6 B#4 h=1 facets  2 4* 5 7 det=1
0 1 1 2 1  * 1 0 0 0 0
This indicates that the ray is incident to the origin. Alternatively, if the allbases option is used, all cobases will be printed out.
V-representation: If the input is a V-representation, the cobasis is a list of the input vertices /rays that define the current facet. For example, with input file cube.ext a typical output is:
V#1 F#3 B#4 h=2 vertices/rays  2 3 4 5 det=8
1 0 0 -1
Here the output V#1 can be ignored. There have been 3 facets output up to this point, and 4 bases have been computed. This facet is defined by the vertices in positions 2,3,4,5 in the input file.

restart  V# R# B# depth {facet #s or vertex/ray #s}

lrs can be restarted from any known cobasis. The calculation will proceed to normal termination. All of the information is contained in the output from a printcobasis option.  The order of the indices is very important, enter them exactly as they appear in the output from the previously aborted run. To restart from cobasis:

V#5 R#0 B#5 h=1 facets  3 4 5 det=1
enter:
restart 5 0 5 1 3 4 5

startingcobasis i1 i2 i ... in-1

This allows the user to specify a known cobasis for beginning the reverse search. i1 i2 i ... in-1 is a list of the inequalities (for H-representation) or vertices/rays (for V-representation) that define a cobasis. If it is invalid, or this option is not specified, lrs will find its own starting cobasis. For example, with cube.ine, the user can start at vertex (-1,1,-1) by specifying:
startingcobasis 1 3 4

volume                                              // V-representation  only //

voronoi                                              // V-representation  only //

### Estimation

The estimation feature of lrs allows estimates to be made of the output size and running time. These are based on Knuth's technique for estimating the size of backtracking trees, and are described in Avis and Devroye(1994).  The estimate is unbiased, that is the expected value of the estimate is the actual value. To get an estimate use the maxdepth option to limit the search depth, and the estimates option:

maxdepth d
estimates k

This will cause lrs to perform k random probes from each  node of the tree at depth d. k should be at least 1 and d at least zero.

H-representation: If the input is an H-representation, the program gives an unbiased estimate of the number of vertices and  rays in the V-representation,  and the total number of bases that will be computed by lrs. For the H-representation cube.ine, the options maxdepth 1 and estimates 1 produce the output:
*Estimates: vertices=9    rays=0    bases=9
*Total number of tree nodes evaluated: 6
In this case the V-representation of the cube is estimated to have 9 vertices, and it is estimated that lrs will compute a total of 9 bases. The estimate was based on evaluating 6 tree nodes.  Note: The estimate for the number of rays may be an overestimate if the polyhedron is not a cone, since some rays may be duplicated in the output - see subsection Output Duplication.

V-representation: If the input is a V-representation, the program gives an unbiased estimate of the number of facets in the H-representation, and the total number of bases that lrs will compute.  For V-representation cube.ext, the options maxdepth 0 and estimates 3 produce the output:
*Estimates:  facets=7  bases=8
*Total number of tree nodes evaluated: 7
In this cases it is estimated that the H-representation of the cube will contain 7 facets, and it is estimated that  lrs will compute a total of 7 bases to find it. The estimate was formed by evaluating 7 tree nodes. Note: The number of facets estimated may be an overestimate if the polyhedron is not bounded - see subsection Output Duplication.

Voronoi diagrams: Estimates for the number of Voronoi vertices and Voronoi rays for a V-representation of a set of data points may be obtained by combining the voronoi, estimates and maxdepth options.

Repeated estimates: In order to get estimates with different random probes, lrs can be given a seed for the random number generator. There are two ways: an option and a command line argument.

seed n

The integer n is used as a seed for the random number generator.

The command line argument is an integer n which will be the seed and overrides a seed given as an option.

lrs n < filename

Using the estimator: The running time of lrs is proportional to the number of bases, so an estimate of the number of bases gives an easy way to estimate the running time for solving the complete problem by lrs. For the case of polytopes that contain the origin, a V-representation can be processed as an H-representation and vice-versa (this is an application of duality). Hence facet estimates for a V-representation can also be obtained by running the problem as an H-representation with the estimates option. The estimated number of vertices will be in this case be an unbiased estimate for the number of facets for the original problem. So running the estimator on the file cube.ext with the header set as H-representation , maxdepth 0 and estimates 3, we get the output:
*Estimates: vertices=5    rays=0    bases=9
*Total number of tree nodes evaluated: 8
Since the origin is an interior point, the estimated number of vertices is an accurate estimate of the number of facets of the H-representation of the cube.   Similarly, estimates for an input  H-representation of a polytope containing the origin may be obtained by processing the file as a V-representation. The output will be essentially the same, but the number of bases computed may be very different, see the subsection H- vs V-representation.  For a large problem of this type, it is useful to get estimates for the number of bases that lrs will compute for both V- and H-representations, so that simpler problem can be chosen.

The estimates may also be used to judge the feasibility of solving the problem using other codes. For example, any code that uses triangulation/perturbation to resolve degeneracy will have trouble if the number of bases is huge. Codes which must store all the output in memory (currently all codes except reverse search methods such as lrs) will have trouble if the estimated output size is huge.

### Linear Programming

lrs can be used to solve linear programming problems in rational arithmetic when the input is an H-representation.  The option:

maximize a0 a1 ... an-1                                           // H-representation  only //

simply maximizes the function  a0 + a1 x1 + ... + an-1 xn-1
over the given polyhedron. A optimal vertex is given when it exists, otherwise for unbounded solutions a vertex and ray is given. A minimization will be performed if the  following option is specified:

minimize a0 a1 ... an-1                                             // H-representation  only //

### Volume Computation

lrs can be used to compute the volume of a polytope given as a V-representation. The option

volume                                                                            // V-representation only //

will cause the volume to be computed. For input cube.ext, the output is:
*Volume=8
If the volume option is applied to an H-representation, the results are not predictable.

For polytopes given by a H-representation, it will first be necessary to compute the V-representation. Alternatively the program pd  may be used, which works directly with the H-representation.

### Voronoi Diagrams

lrs can be used the compute the V-vertices of a Voronoi diagram of a set of data points in n-1 dimensional space. To do this we use a standard lifting procedure (see, e.g., Edelsbrunner, "Algorithms in Combinatorial Geometry," pp 296-297) . Each point is mapped to a half space tangent to the parabaloid in n dimensions, by the mapping:

p , p , ...., pn-1     ->    (p1 +   p2+  ...   +  pn-1 ) - 2 p1  x - 2 p x- .... - 2  pn-1 xn -1  + x n>= 0

lrs is applied to the H-representation so created.  This transformation is performed automatically for a V-representation if the

voronoi                     // V-representation only //

option is specified.
Note: The input file must consist entirely of data points (no rays), i.e.. there must be a one in column one of each line. The volume option should not be used, since the volume reported will not be the volume of the original V-representation.
The output will consist of the Voronoi vertices (columns beginning with a one) and Voronoi rays (columns beginning with zero) for the Voronoi diagram defined on the data points.  If the printcobasis option is given, the n "data points" indices produced will tell which set of input data points corresponds to the given Voronoi vertex or ray. In case of degeneracies, a given Voronoi vertex may be generated by more than n of the input data points. In this case, use of the allbases option will cause all  sets of n input data points corresponding to a Voronoi vertex to be printed. For Voronoi rays, the immediately preceding  cobasis is the cobasis of the the Voronoi vertex from which the ray emanates.  The index followed by a * is the data point to drop in order to generate the ray. If the geometric option is given the correspondence between Voronoi rays and Voronoi vertices will be produced automatically.

Example: Compute the Voronoi diagram of the planar point set (0,0), (2,1), (1,2), (0,4), (4,0), (4,4) (2,-4).
vor7-3
*6 Voronoi vertices and 5 rays
*7 input data points
V-representation
begin
7 3 integer
1 0 0
1 2 1
1 1 2
1 0 4
1 4 0
1 4 4
1 2 -4
end
voronoi
printcobasis
allbases
geometric

The output produced is

V-representation
begin
***** 3 rational
V#1 R#0 B#1 h=0 data points  1 5 7 det=64
1 2 -3/2
V#1 R#1 B#1 h=0 data points  1 5* 7 det=64
0 -2 -1  * 1 2 -3/2
V#1 R#2 B#1 h=0 data points  1* 5 7 det=64
0 2 -1  * 1 2 -3/2
V#1 R#2 B#2 h=1 data points  1 2 5 det=16
1 2 -3/2
V#2 R#2 B#3 h=2 data points  1 2 3 det=12
1 5/6 5/6
V#3 R#2 B#4 h=3 data points  1 3 4 det=16
1 -3/2 2
V#3 R#3 B#4 h=3 data points  1 3* 4 det=16
0 -1 0  * 1 -3/2 2
V#4 R#3 B#5 h=2 data points  2 5 6 det=32
1 15/4 2
V#4 R#4 B#5 h=2 data points  2* 5 6 det=32
0 1 0  * 1 15/4 2
V#5 R#4 B#6 h=3 data points  2 3 6 det=20
1 27/10 27/10
V#6 R#4 B#7 h=4 data points  3 4 6 det=32
1 2 15/4
V#6 R#5 B#7 h=4 data points  3* 4 6 det=32
0 0 1  * 1 2 15/4
end

The output contains 6 Voronoi vertices :
(2, -3/2), (5/6,5/6),(-3/2,2),(15/4,2), (27/10,27/10), (2,15/4).
The Voronoi vertex (2,-3/2) appears twice in the output with data point indices 1 5 7 and 1 2 5. This means that it is degenerate and is defined by the set of 4 input data point in positions 1,2,5,7 in the input file. I.e.. it is the centre of an empty  circle through the four input data points (0,0), (2,1), 4,0), (2,-4).  The other Voronoi vertices appear once each and are defined respectively by the data points with indices (i.e..  position in the input file)  1 2 3,  1 3 4,  2 5 6,  2 3 6 and 3 4 6. The  Voronoi diagram has 5 rays
(2, -3/2) + (-2t,-t),    (2,-3/2)+(2t,-t),    (-3/2,2)+(-t,0),    (15/4,2)+(t,0),    (2,15/4)+(0,t)

For example, the first ray in the output appears:
V#1 R#1 B#1 h=0 data points  1 5* 7 det=64
0 -2 -1  * 1 2 -3/2
This means that the ray (-2t,-t) emanates from the vertex defined by data points 1 5 7, namely (2, -3/2). The asterisk on index 5 indicates that the ray is defined by the data points with indices 5 and 7, namely (0,0) and (2,-4).

### Extreme Point Enumeration and Redundant Inequalities

A convex hull problem that occurs frequently is to enumerate the extreme points (vertices) of a given set of input points. This problem is in fact much simpler than the problem of finding the facets of the given input point set. It can be solved by linear programming.  The dual problem is to remove redundant inequalities from an H-representation. An input  inequality is redundant if it can be deleted without changing the polyhedron. lrs does not solve these problems , but they can be solved by cdd using the vertex_listing  and facet_listing  options. Alternatively, two programs that perform this task are contained in the ftp site: redund.c and build.c . To use these programs compile them with the commands:

gcc -O -o build build.c                  gcc -O -o redund redund.c

To remove input points that are not vertices from a V-representation or redundant inequalities from an H-representation use the command:

build  < filename | redund

The resulting file can be used directly with lrs, or even piped into lrs. In fact, lrs works best if the input is non-redundant, see the section Redundancy vs Degeneracy.

Warning: The origin will not be removed from a V-representation even if it is not a vertex.

### Timing and Interrupts

lrs handles certain signals unless it is compiled with the -DOMIT_SIGNALS option. It is possible to interrupt lrs and get the latest cobasis, which can be used for restarting the program (useful if the machine is going down!)
signal                                operation
USR1                         print current cobasis and continue
TERM                       print current cobasis and terminate
INT (ctrl-C)                      ditto
HUP                                      ditto
lrs also provides timing information, unless compiled with the option -DOMIT_TIMES.

### Error Messages and Troubleshooting

The most common error occurs from an incorrect input file specification, please check the section File Formats carefully. In particular, lrs does not check the type or number of input coefficients specified.  After the line
m n rational
you must specify exactly m*n rational or integer coefficients. They are read  in free format, but normally each input facet or vertex/ray is begun on a new line.  See note for cdd users.

The following error messages are produced by lrs. They are  arranged in alphabetic order.

Data type must be integer of rational

lrs does not handle floating point data, change to integer or rational input.

Digits must be at most 2295  Change MAX_DIGITS and recompile

The digits option was specified, but the number of  decimal digits is too large (the values shown here is for 64 bit machines). MAX_DIGITS  (default 255) is the maximum number of array locations used for extended precision arithmetic. This value should be increased  and lrs recompiled.

Input Polyhedron does not have full dimension
If input is a cone, change to H-representation, or add the origin 1 0 0 ... 0

The input polyhedron does not have dimension n-1. Either there is a mistake in the input, or it must be projected onto a full dimensional subspace. cdd has a function for doing this. The second message only appears for V-representations - note that at least one vertex must be supplied, or else the problem can be run as a H-representation.

Invalid Co-basis - does not have correct rank

An attempt to restart from an invalid cobasis has been made. Check that the indices are entered exactly as they appear in the previous aborted run.

Maximize/minimize only valid for H-representation

LP operations can only be performed on H-representations.

No begin line

No data in file

No feasible solution

The input is an H-representation of an infeasible system.

Starting cobasis indices must be distinct and in range 1 .. m

The startingcobasis option has been used, but the indices supplied were not valid: i.e. distinct numbers between 1..m.

Trying to restart from infeasible dictionary

An attempt to restart from an invalid cobasis has been made. Check that the indices are entered exactly  in the order appear in the previous aborted run.

#### H- vs V- representation

lrs is programmed to manipulate H-representations directly. A file presented as a V-representation is processed by lifting it to a cone in one higher dimension, which is treated internally as a H-representation. If the input file is a polytope which contains the origin, then the user has two options. Submit it as a V-representation and have it processed as just described, or submit it as a H-representation, and interpret the output as a list of facet inequalities rather than "vertices". Since this will not be lifted, it will be processed in a different way by lrs. Sometimes a degenerate V-representation may run more quickly as a H-representation, and sometimes more slowly. To decide which representation to use for a large problem, the user can run the estimates option and choose the representation with fewest estimated bases.

#### Redundancy vs Degeneracy

For an H-representation, an input is redundant if some inequality can be deleted without changing the polyhedron. It is degenerate if (in d dimensions) at least one vertex lies on d+1 or more facets.  Similarly in a V-representation an input is redundant if some input point is not a vertex of the convex hull.  It is degenerate if some facet contains d+1 or more input points. Degeneracy causes pivot  or triangulation based methods such as lrs to  run slowly. Redundancy is one cause of degeneracy, but it can be avoided by pre-processing the input files. See section Extreme Point Enumeration and Redundant Inequalities for instructions on how to do this. This pre-processing is unnecessary if it is known that the input is non-redundant.

Even with redundant input removed a polyhedron may be highly degenerate. In directory ine/metric there are many highly degenerate combinatorial polytopes. These are difficult problems for all vertex enumeration/convex hull programs that use pivoting, such as lrs.  For example, the file ccc7.ine is a cone with 63 facets in 21 dimensions. It has 38,780 extreme rays, but computing these required the evaluation of 247,271,659 bases!

#### Memory considerations

The strong point of lrs is that it does not save the output produced, so in theory it cannot run out of memory.  With cache size one all memory is allocated at the beginning, so if lrs starts running it will not run out of memory. It is possible however that the number of digits required to do the calculations exceeds the amount specified on the digits option, or the default. In practice, this problem will also arise early in the computation. In any case, a message is printed and the calculation can be restarted. In order to improve performance, some dictionaries should be cached. The default of 10 can be overridden by the cache option. If the dictionary is in the cache it does not need to be recomputed when backtracking, reducing  processing time by about 40%. Since the cache is allocated dynamically, a cache size that is too large can potentially use up large ammounts of machine memory.

#### Geometric Rays

A minimum V-representation of a polyhedron is a minimum set of vertices and rays such that each point in the polyhedron can be expressed as a convex combination of vertices plus a non-negative combination of rays. For the cube, if we delete the inequality
x3 <= 1, i.e.. the line 1 0 0 -1 from file cube.ine, we get the output:
V-representation
***** 4 rational
1 1 1 -1
0 0 0 1
1 -1 1 -1
1 1 -1 -1
1 -1 -1 -1
end
indicating the polyhedron is the convex combination of 4 vertices and 1 ray. With the geometric option, we get the output:
V-representation
begin
***** 4 rational
1 1 1 -1
0 0 0 1  * 1 1 1 -1
1 -1 1 -1
0 0 0 1  * 1 -1 1 -1
1 1 -1 -1
0 0 0 1  * 1 1 -1 -1
1 -1 -1 -1
0 0 0 1  * 1 -1 -1 -1
end
This indicates that geometrically, the polyhedron has 4 parallel extreme rays (0,0,t) , one incident to each vertex. With the geometric option, all rays will be printed. Without the option, lrs tries to print each ray once, but in some cases duplicates will remain, see  subsection Output Duplication.

Output Duplication

For degenerate inputs, pivot based methods such as lrs may generate the same output vertex/ray/facet many times. Unless the allbases option is specified, lrs makes checks in order to remove duplicates.  An output is only printed when it occurs with a lexicographically minimum basis. This removes all duplicate vertices, but rays/facets may still be output more than once. This is due to the fact that duplicate geometric rays cannot always be detected. A warning message is produced when duplicates may occur in the output. They can be removed using the program buffer.c. Two important types of input never produce duplicate output: polytopes (i.e. bounded polyhedra) and cones (i.e. polyhedra where the origin is the only vertex).

### Acknowledgements and References

I would like to thank many people for helping with this implementation project. Komei Fukuda encouraged me from the start, collaborated in designing the file formats, and provided many suggestions for improving the code. David Bremner implemented memory allocation, caching and signals. Jerry Quinn coded the integer divide routine. Bug reports were provided by many users, for which I thank them. In particular Gerardo Garbulsky's extensive use of earlier versions suggested many refinements, Ambros Marzetta demonstrated the importance of caching and Andreas Enge helped debug the volume computation.

D. Avis, "A C Implementation of the Reverse Search Vertex Enumeration Algorithm," in RIMS Kokyuroku 872, ed. H. Imai, Kyoto University (May 1994). ftp://mutt.cs.mcgill.ca/pub/doc/avis/Av94a.ps.gz

D. Avis, D. Bremner, and R. Seidel, "How Good are Convex Hull Algorithms?," Computational Geometry: Theory and Applications, Vol 7,pp.265-301(1997). ftp://mutt.cs.mcgill.ca/pub/doc/avis/ABS96a.ps.gz

D. Avis and L. Devroye, "Estimating the Number of Vertices of a Polyhedron," pp. 179-190 in Snapshots of Computational and Discrete Geometry, ed. D. Avis and P. Bose, School of Computer Science, McGill University (1994). ftp://mutt.cs.mcgill.ca/pub/doc/avis/AD94a.ps.gz

D. Avis and K. Fukuda, "A Pivoting Algorithm for Convex Hulls and Vertex Enumeration of Arrangements and Polyhedra," Discrete and Computational Geometry, Vol. 8, pp. 295-313 (1992).  ftp://mutt.cs.mcgill.ca/pub/doc/avis/AF92b.ps.gz

D. Bremner, K. Fukuda and A. Marzetta, Primal-Dual Methods for Vertex and Facet Enumeration, 13th ACM
Symposium on Computational Geometry SCG 1997, 49-56.