5.1.98 mres

Syntax:

mres ( ideal_expression, int_expression )
mres ( module_expression, int_expression )

Type:

resolution

Purpose:

computes a minimal free resolution of an ideal or module M with the standard basis method. More precisely, let A=matrix(M), then mres computes a free resolution of

where the columns of the matrix are a minimal set of generators of M if the basering is local or if M is homogeneous. If the int expression k is not zero, then the computation stops after k steps and returns a list of modules , i=1...k.
mres(M,0) returns a resolution consisting of at most n+2 modules, where n is the number of variables of the basering. Let list L=mres(M,0); then L[1] consists of a minimal set of generators of the input, L[2] consists of a minimal set of generators for the first syzygy module of L[1], etc., until L[p+1], such that for , but L[p+1], the first syzygy module of L[p], is 0 (if the basering is not a qring).

Note:

Accessing single elements of a resolution may require some partial computations to be finished and may therefore take some time.

Note:

mres honours degBound.

Example:

ring r=31991,(t,x,y,z,w),ls; ideal M=t2x2+tx2y+x2yz,t2y2+ty2z+y2zw, t2z2+tz2w+xz2w,t2w2+txw2+xyw2; resolution L=mres(M,0); L; ==> 1 4 15 18 7 1 ==> r <-- r <-- r <-- r <-- r <-- r ==> ==> 0 1 2 3 4 5 ==> // projective dimension of M is 5