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Matroids :: relaxation

relaxation -- relaxation of matroid

Synopsis

Description

Let M = (E, B) be a matroid with bases B. If there is a subset S of E that is both a circuit and a hyperplane of M, then the set $B \cup \{S\}$ is the set of bases of a matroid on E, called the relaxation of M by S.

If no set S is provided, then this function will take S to be a random circuit-hyperplane (the first in lexicographic order). If no circuit-hyperplanes exist, then an error is produced.

Many interesting matroids arise as relaxations of other matroids: e.g. the non-Fano matroid is a relaxation of the Fano matroid, and the non-Pappus matroid is a relaxation of the Pappus matroid.

i1 : P = specificMatroid "pappus"

o1 = a "matroid" of rank 3 on 9 elements

o1 : Matroid
i2 : NP = specificMatroid "nonpappus"

o2 = a "matroid" of rank 3 on 9 elements

o2 : Matroid
i3 : NP == relaxation(P, set{6,7,8})

o3 = true

Ways to use relaxation :

For the programmer

The object relaxation is a method function with options.