# what is a Groebner basis?

A Groebner basis is a specific generating set of an ideal or submodule over a polynomial ring. It is not minimal in general, but has extremely nice properties; it is reasonably easy to extract information about the ideal or submodule from a Groebner basis. We first describe Groebner bases in the important special case of an ideal in a polynomial ring. We will then describe Groebner bases of submodules, and over more general rings.

Let $R = k[x_1, ..., x_n]$ be a polynomial ring over a field $k$, and let $I \subset R$ be an ideal. A monomial order on $R$ is a total order, $>$, on the monomials of $R$, which satisfies two conditions: (1) $m > 1$, for every monomial $m \neq 1$, and (2) the order is multiplicative: $m > n$ implies that $mp > np$, for all monomials $m$, $n$, $p$.

In Macaulay2, each ring has a monomial order (also called a term order) associated with it. The default monomial order is GRevLex. See monomial orderings for more information.

Given a term order, the leading term is the one whose monomial is greatest in this order.

 i1 : R = ZZ/1277[a..d] o1 = R o1 : PolynomialRing i2 : F = 5*a^3 + d^2 + a*d + b*c + 1 3 2 o2 = 5a + b*c + a*d + d + 1 o2 : R i3 : leadTerm F 3 o3 = 5a o3 : R
For an ideal $I \subset R$, the initial ideal $in(I)$ is the ideal generated by the leading terms of the elements in I. A Groebner basis for I is a set of generators for I whose leading terms generate $in(I)$.

 i4 : R = ZZ/1277[x,y]; i5 : I = ideal(x^3 - 2*x*y, x^2*y - 2*y^2 + x); o5 : Ideal of R i6 : leadTerm I o6 = | y2 xy x2 | 1 3 o6 : Matrix R <--- R i7 : gens gb I o7 = | y2+638x xy x2 | 1 3 o7 : Matrix R <--- R
The above example also shows that the leading terms of any set of generators of I do not necessarily generate in(I).

A Groebner basis for an ideal depends on the monomial ordering used in the ring .

 i8 : R = ZZ/1277[x,y, MonomialOrder => Lex]; i9 : I = ideal(x^3 - 2*x*y, x^2*y - 2*y^2 + x); o9 : Ideal of R i10 : gens gb I o10 = | y3 x-2y2 | 1 2 o10 : Matrix R <--- R