7.3.15 mres (plural)

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 Groebner basis method. More precisely, let A=matrix(M), then mres computes a free resolution of

where the columns of the matrix are a (possibly) minimal set of generators of . If the int expression k is not zero, then the computation stops after k steps and returns a resolution consisting of modules , .
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 M, 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 that some partial computations have to be finished and may therefore take some time. Hence, assigning right away to a list is the recommended way to do it.

Example:

LIB "ncalg.lib"; def A = makeUsl2(); setring A; // this algebra is U(sl_2) option(redSB); option(redTail); ideal i = e,f,h; i = std(i); resolution M=mres(i,0); M; ==> 1 2 2 1 ==> A <-- A <-- A <-- A ==> ==> 0 1 2 3 ==> list l = M; l; ==> [1]: ==> _[1]=f ==> _[2]=e ==> [2]: ==> _[1]=ef*gen(1)-f2*gen(2)-2h*gen(1)-2*gen(1) ==> _[2]=e2*gen(1)-ef*gen(2)-h*gen(2)+2*gen(2) ==> [3]: ==> _[1]=e*gen(1)-f*gen(2) // see the exactness at this point size(ideal(transpose(matrix(l[2]))*transpose(matrix(l[1])))); ==> 0 print(matrix(M[3])); ==> e, ==> -f // see the exactness at this point size(ideal(transpose(matrix(l[3]))*transpose(matrix(l[2])))); ==> 0