# 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.

Over $\mathbb Z$, the basic idea of multivariate polynomial division with remainder in OSCAR is as follows: If $ax^\alpha$ is the leading term of the intermediate dividend, $f_i$ is *some* divisor whose leading monomial equals $x^\alpha$, say $\text{LT}(f_i) = bx^\alpha$, and $r$ is the remainder of $a$ on division by $b$ in $\mathbb Z$, then $ax^\alpha$ is replaced by $rx^\alpha$.

##### 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.

Over $\mathbb Z$, the standard bases returned by OSCAR are strong in the sense above.

##### 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])
```