# Commutative rings¶

class sage.categories.commutative_rings.CommutativeRings(base_category)

The category of commutative rings

commutative rings with unity, i.e. rings with commutative * and a multiplicative identity

EXAMPLES:

sage: C = CommutativeRings(); C
Category of commutative rings
sage: C.super_categories()
[Category of rings, Category of commutative monoids]

class CartesianProducts(category, *args)
extra_super_categories()

Let Sage knows that Cartesian products of commutative rings is a commutative ring.

EXAMPLES:

sage: CommutativeRings().Commutative().CartesianProducts().extra_super_categories()
[Category of commutative rings]
sage: cartesian_product([ZZ, Zmod(34), QQ, GF(5)]) in CommutativeRings()
True

class ElementMethods

Bases: object

class Finite(base_category)

Check that Sage knows that Cartesian products of finite commutative rings is a finite commutative ring.

EXAMPLES:

sage: cartesian_product([Zmod(34), GF(5)]) in Rings().Commutative().Finite()
True

class ParentMethods

Bases: object

cyclotomic_cosets(q, cosets=None)

Return the (multiplicative) orbits of q in the ring.

Let $$R$$ be a finite commutative ring. The group of invertible elements $$R^*$$ in $$R$$ gives rise to a group action on $$R$$ by multiplication. An orbit of the subgroup generated by an invertible element $$q$$ is called a $$q$$-cyclotomic coset (since in a finite ring, each invertible element is a root of unity).

These cosets arise in the theory of minimal polynomials of finite fields, duadic codes and combinatorial designs. Fix a primitive element $$z$$ of $$GF(q^k)$$. The minimal polynomial of $$z^s$$ over $$GF(q)$$ is given by

$M_s(x) = \prod_{i \in C_s} (x - z^i),$

where $$C_s$$ is the $$q$$-cyclotomic coset mod $$n$$ containing $$s$$, $$n = q^k - 1$$.

Note

When $$R = \ZZ / n \ZZ$$ the smallest element of each coset is sometimes called a coset leader. This function returns sorted lists so that the coset leader will always be the first element of the coset.

INPUT:

• q – an invertible element of the ring

• cosets – an optional lists of elements of self. If provided, the function only return the list of cosets that contain some element from cosets.

OUTPUT:

A list of lists.

EXAMPLES:

sage: Zmod(11).cyclotomic_cosets(2)
[, [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]]
sage: Zmod(15).cyclotomic_cosets(2)
[, [1, 2, 4, 8], [3, 6, 9, 12], [5, 10], [7, 11, 13, 14]]


Since the group of invertible elements of a finite field is cyclic, the set of squares is a particular case of cyclotomic coset:

sage: K = GF(25,'z')
sage: a = K.multiplicative_generator()
sage: K.cyclotomic_cosets(a**2,cosets=)
[[1, 2, 3, 4, z + 1, z + 3,
2*z + 1, 2*z + 2, 3*z + 3,
3*z + 4, 4*z + 2, 4*z + 4]]
sage: sorted(b for b in K if not b.is_zero() and b.is_square())
[1, 2, 3, 4, z + 1, z + 3,
2*z + 1, 2*z + 2, 3*z + 3,
3*z + 4, 4*z + 2, 4*z + 4]


We compute some examples of minimal polynomials:

sage: K = GF(27,'z')
sage: a = K.multiplicative_generator()
sage: R.<X> = PolynomialRing(K, 'X')
sage: a.minimal_polynomial('X')
X^3 + 2*X + 1
sage: cyc3 = Zmod(26).cyclotomic_cosets(3,cosets=); cyc3
[[1, 3, 9]]
sage: prod(X - a**i for i in cyc3)
X^3 + 2*X + 1

sage: (a**7).minimal_polynomial('X')
X^3 + X^2 + 2*X + 1
sage: cyc7 = Zmod(26).cyclotomic_cosets(3,cosets=); cyc7
[[7, 11, 21]]
sage: prod(X - a**i for i in cyc7)
X^3 + X^2 + 2*X + 1


Cyclotomic cosets of fields are useful in combinatorial design theory to provide so called difference families (see Wikipedia article Difference_set and difference_family). This is illustrated on the following examples:

sage: K = GF(5)
sage: a = K.multiplicative_generator()
sage: H = K.cyclotomic_cosets(a**2, cosets=[1,2]); H
[[1, 4], [2, 3]]
sage: sorted(x-y for D in H for x in D for y in D if x != y)
[1, 2, 3, 4]

sage: K = GF(37)
sage: a = K.multiplicative_generator()
sage: H = K.cyclotomic_cosets(a**4, cosets=); H
[[1, 7, 9, 10, 12, 16, 26, 33, 34]]
sage: sorted(x-y for D in H for x in D for y in D if x != y)
[1, 1, 2, 2, 3, 3, 4, 4, 5, 5, ..., 33, 34, 34, 35, 35, 36, 36]


The method cyclotomic_cosets works on any finite commutative ring:

sage: R = cartesian_product([GF(7), Zmod(14)])
sage: a = R((3,5))
sage: R.cyclotomic_cosets((3,5), [(1,1)])
[[(1, 1), (2, 11), (3, 5), (4, 9), (5, 3), (6, 13)]]

class ParentMethods

Bases: object

over(base=None, gen=None, gens=None, name=None, names=None)

Return this ring, considered as an extension of base.

INPUT:

• base – a commutative ring or a morphism or None (default: None); the base of this extension or its defining morphism

• gen – a generator of this extension (over its base) or None (default: None);

• gens – a list of generators of this extension (over its base) or None (default: None);

• name – a variable name or None (default: None)

• names – a list or a tuple of variable names or None (default: None)

EXAMPLES:

We construct an extension of finite fields:

sage: F = GF(5^2)
sage: k = GF(5^4)
sage: z4 = k.gen()

sage: K = k.over(F)
sage: K
Field in z4 with defining polynomial x^2 + (4*z2 + 3)*x + z2 over its base


If not explicitly given, the default generator of the top ring (here k) is used and the same name is kept:

sage: K.gen()
z4
sage: K(z4)
z4


However, it is possible to specify another generator and/or another name. For example:

sage: Ka = k.over(F, name='a')
sage: Ka
Field in a with defining polynomial x^2 + (4*z2 + 3)*x + z2 over its base
sage: Ka.gen()
a
sage: Ka(z4)
a

sage: Kb = k.over(F, gen=-z4+1, name='b')
sage: Kb
Field in b with defining polynomial x^2 + z2*x + 4 over its base
sage: Kb.gen()
b
sage: Kb(-z4+1)
b


Note that the shortcut K.<a> is also available:

sage: KKa.<a> = k.over(F)
sage: KKa is Ka
True


Building an extension on top of another extension is allowed:

sage: L = GF(5^12).over(K)
sage: L
Field in z12 with defining polynomial x^3 + (1 + (4*z2 + 2)*z4)*x^2 + (2 + 2*z4)*x - z4 over its base
sage: L.base_ring()
Field in z4 with defining polynomial x^2 + (4*z2 + 3)*x + z2 over its base


The successive bases of an extension are accessible via the method sage.rings.ring_extension.RingExtension_generic.bases():

sage: L.bases()
[Field in z12 with defining polynomial x^3 + (1 + (4*z2 + 2)*z4)*x^2 + (2 + 2*z4)*x - z4 over its base,
Field in z4 with defining polynomial x^2 + (4*z2 + 3)*x + z2 over its base,
Finite Field in z2 of size 5^2]


When base is omitted, the canonical base of the ring is used:

sage: S.<x> = QQ[]
sage: E = S.over()
sage: E
Univariate Polynomial Ring in x over Rational Field over its base
sage: E.base_ring()
Rational Field


Here is an example where base is a defining morphism:

sage: k.<a> = QQ.extension(x^2 - 2)
sage: l.<b> = QQ.extension(x^4 - 2)
sage: f = k.hom([b^2])
sage: L = l.over(f)
sage: L
Field in b with defining polynomial x^2 - a over its base
sage: L.base_ring()
Number Field in a with defining polynomial x^2 - 2


Similarly, one can create a tower of extensions:

sage: K = k.over()
sage: L = l.over(Hom(K,l)(f))
sage: L
Field in b with defining polynomial x^2 - a over its base
sage: L.base_ring()
Field in a with defining polynomial x^2 - 2 over its base
sage: L.bases()
[Field in b with defining polynomial x^2 - a over its base,
Field in a with defining polynomial x^2 - 2 over its base,
Rational Field]