# Difference between revisions of "ApCoCoA-1:NC.MB"

Line 23: | Line 23: | ||

<example> | <example> | ||

Use ZZ/(2)[t,x,y]; | Use ZZ/(2)[t,x,y]; | ||

− | NC.SetOrdering( | + | NC.SetOrdering(<quotes>LLEX</quotes>); |

F1 := [[x^2], [y,x]]; -- x^2+yx | F1 := [[x^2], [y,x]]; -- x^2+yx | ||

F2 := [[x,y], [t,y]]; -- xy+ty | F2 := [[x,y], [t,y]]; -- xy+ty |

## Revision as of 18:01, 14 May 2013

## NC.MB

Enumerate a Macaulay's basis of a finitely generated `K`-algebra.

Given a two-sided ideal `I` in a non-commutative polynomial ring `P` over `K`, we can consider the `K`-algebra `P/I` as a `K`-vector space. Moreover, let `G` be a Groebner basis of `I`, and let `B` be the set of all words which are not a multiple of any word in the leading word set `LW{G}`. Then the residue class of the words in `B` form a `K`-basis, called a *Macaulay's basis*, of `P/I`. For the sake of computing the values of the Hilbert function (see NC.HF) of `P/I`, in this function we require that `G` has to be a Groebner basis with respect to a length compatible word ordering (see NC.SetOrdering).

### Syntax

NC.MB(G:LIST[, DB:INT]):LIST

### Description

*Please note:* The function(s) explained on this page is/are using the *ApCoCoAServer*. You will have to start the ApCoCoAServer in order to use it/them.

Please set non-commutative polynomial ring (via the command Use) and word ordering (via the function NC.SetOrdering) before calling this function. The default word ordering is the length-lexicographic ordering ("LLEX"). For more information, please check the relevant commands and functions.

@param

*G:*a LIST of non-zero non-commutative polynomials, which form a Groebner basis with respect to a length compatible word ordering. Each polynomial is represented as a LIST of LISTs, and each element in every inner LIST involves only one indeterminate or none (a constant). For example, the polynomial`f=2x[2]y[1]x[2]^2-9y[2]x[1]^2x[2]^3+5`is represented as F:=[[2x[1],y[1],x[2]^2], [-9y[2],x[1]^2,x[2]^3], [5]]. The zero polynomial`0`is represented as the empty LIST [].*Warning:*users should take responsibility to ensure that G is indeed a Groebner basis with respect to a length compatible word ordering!@return: a LIST of words forming a Macaulay's basis of the K-algebra

`P/<G>`.

Optional parameter:

@param

*DB:*a positive INT, which is a degree bound of the lengths of words.*Note that*we set DB=32 by default. Thus, in the case that`P/<G>`has a finite Macaulay's basis, it is necessary to set DB to a large enough INT in order to compute the whole Macaulay's basis.

#### Example

Use ZZ/(2)[t,x,y]; NC.SetOrdering(<quotes>LLEX</quotes>); F1 := [[x^2], [y,x]]; -- x^2+yx F2 := [[x,y], [t,y]]; -- xy+ty F3 := [[x,t], [t,x]]; -- xt+tx F4 := [[y,t], [t,y]]; -- yt+ty G := [F1, F2,F3,F4]; Gb:=NC.GB(G); NC.MB(Gb,3); [[[1]], [[y], [x], [t]], [[y, t], [y, x], [y^2], [x, t], [t^2]], [[y^3], [y^2, x], [y^2, t], [x, t^2], [t^3]]] -------------------------------

### See also