Macaulay2 » Documentation
Packages » EdgeIdeals :: isSCM
next | previous | forward | backward | up | index | toc

isSCM -- determines if a (hyper)graph is sequentially Cohen-Macaulay

Synopsis

Description

This uses the edge ideal notion of sequential Cohen-Macaulayness; a hypergraph is called SCM if and only if its edge ideal is SCM. The algorithm is based on work of Herzog and Hibi, using the Alexander dual. H is SCM if and only if the Alexander dual of the edge ideal of H is componentwise linear.

There is an optional argument called Gins for isSCM. The default value is false, meaning that isSCM takes the Alexander dual of the edge ideal of H and checks in all relevant degrees to see if the ideal in that degree has a linear resolution. In characteristic zero with the reverse-lex order, one can test for componentwise linearity using gins, which may be faster in some cases. This approach is based on work of Aramova-Herzog-Hibi and Conca. We make no attempt to check the characteristic of the field or the monomial order, so use caution when using this method.

i1 : R = QQ[a..f];
 
i2 : G = cycle(R,4)

o2 = Graph{"edges" => {{a, b}, {b, c}, {c, d}, {a, d}}}
           "ring" => R
           "vertices" => {a, b, c, d, e, f}

o2 : Graph
 
i3 : isSCM G

o3 = false
 
i4 : H = graph(monomialIdeal(a*b,b*c,c*d,a*d,a*e)); --4-cycle with whisker
 
i5 : isSCM H

o5 = true
 
i6 : isSCM(H,Gins=>true) --use Gins technique

o6 = true

See also

Ways to use isSCM :

For the programmer

The object isSCM is a method function with options.