# Difference between revisions of "ApCoCoA-1:NCo.PrefixGB"

## NCo.PrefixGB

Compute a prefix Groebner basis of a finitely generated right ideal in a finitely presented monoid ring.

### Syntax

```NCo.PrefixGB(G:LIST):LIST
```

### Description

Let P=K<X|R> be a finitely presented monoid ring, and let I be a right ideal of P. Given a word ordering Ordering, a set G of non-zero polynomials is called a prefix Groebner basis of I with respect to Ordering if, for each non-zero polynomial f in I, there exists a polynomial g such that the leading word of g is a prefix of the leading word of f. Note that a right ideal in a finitely generated monoid may not have finite prefix Groebner bases. Thus, this function might not terminate.

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 ring environment coefficient field K, alphabet (or set of indeterminates) X, rewrite relations Relations and word ordering Ordering through the functions NCo.SetFp, NCo.SetX, NCo.SetRelations and NCo.SetOrdering, respectively, before using this function. The default coefficient field is the field of rational numbers, i.e. RAT in CoCoAL, and the default ordering is the length-lexicographic ordering "LLEX". For more information, please check the relevant functions.

• @param G: a LIST of non-zero polynomials generating a right ideal in the monoid ring. Each polynomial is represented as a LIST of LISTs, i.e. as [[C1,W1],...,[Cs,Ws]] where, for each i, Wi is a term represented as a STRING and Ci is the coefficient of Wi. For example, polynomial f=xy-y+1 is represented as F:=[[1,"xy"], [-1, "y"], [1,""]]. The zero polynomial 0 is represented as the empty LIST [].

• @return: a LIST of polynomials, which is a prefix Groebner basis of the right ideal generated by G.

#### Example

```NCo.SetX("abc");
NCo.SetOrdering("LLEX");
NCo.SetRelations([["aa",""], ["bb",""], ["ab","c"], ["ac", "b"], ["cb", "a"]]);
F := [[1,"a"],[1,"b"],[1,"c"]];
NCo.PrefixGB([F]);

[[[1, "b"], [-1, ""]], [[1, "a"], [1, "c"], [1, ""]], [[1, "a"], [1, "b"], [1, "c"]],
[[1, "cc"], [1, "b"], [1, "c"]], [[1, "ca"], [1, "a"], [1, ""]], [[1, "bc"], [1, "cc"], [1, "b"]],
[[1, "ba"], [1, "ca"], [1, ""]]]
-------------------------------
```