Gröbner/Standard Bases Over $\mathbb Z$

In this section, we consider a polynomial ring $\mathbb Z[x] = \mathbb Z[x_1, \dots, x_n]$ over the integers. As in the previous section on Gröbner/standard bases over fields, let $>$ be a monomial ordering on $\text{Mon}_n(x)$. With respect to this ordering, the localization $\mathbb Z[x]_>$ and, given a nonzero element $f \in \mathbb Z[x]_>$, the notions leading term, leading monomial, leading exponent, leading coefficient, and tail of $f$ are defined as before.

Examples

julia> R, (x, y) = polynomial_ring(ZZ, [:x, :y]);

julia> reduce(3*x, [2*x])
x

julia> reduce(6*x, [5*x, 2*x])
0

The notion of leading ideals as formulated in the previous section and the definitions of standard bases (Gröbner bases) carry over: A standard basis for an ideal $I\subset K[x]_>$ with respect to $>$ is a finite subset $G$ of $I$ such that $\text{L}_>(G) = \text{L}_>(I)$ (a standard basis with respect to a global monomial ordering is also called a Gröbner basis).

There is, however, a sublety: Over a field, the defining condition of a standard basis as stated above is equivalent to saying that the $\text{LT}_>(g)$, $g\in G\setminus\{0\}$ generate $\text{L}_>(I)$. Over $\mathbb Z$, the latter condition implies the former one, but not vice versa. Consequently, over $\mathbb Z$, a finite subset $G$ of $I$ satisfying the latter condition is called a strong standard basis for $I$ (with respect to $>$).

We refer to the textbook [AL94] for more on this.

Examples

julia> R, (x,y) = polynomial_ring(ZZ, [:x,:y])
(Multivariate polynomial ring in 2 variables over ZZ, ZZMPolyRingElem[x, y])

julia> I = ideal(R, [3*x^2*y+7*y, 4*x*y^2-5*x])
Ideal generated by
  3*x^2*y + 7*y
  4*x*y^2 - 5*x

julia> G = groebner_basis(I, ordering = lex(R))
Gröbner basis with elements
  1: 28*y^3 - 35*y
  2: 4*x*y^2 - 5*x
  3: 15*x^2 + 28*y^2
  4: 3*x^2*y + 7*y
  5: x^2*y^2 - 5*x^2 - 7*y^2
with respect to the ordering
  lex([x, y])