# Rings¶

class sage.categories.rings.Rings(s=None)

Bases: sage.categories.category_singleton.Category_singleton

The category of rings

Associative rings with unit, not necessarily commutative

EXAMPLES:

sage: Rings()
Category of rings
sage: Rings().super_categories()
[Category of rngs, Category of semirings]


TESTS:

sage: TestSuite(Rings()).run()

• Make Rings() into a subcategory or alias of Algebras(ZZ);
• A parent P in the category Rings() should automatically be in the category Algebras(P).
class ElementMethods
is_unit()

Return whether this element is a unit in the ring.

Note

This is a generic implementation for (non-commutative) rings which only works for the one element, its additive inverse, and the zero element. Most rings should provide a more specialized implementation.

EXAMPLES:

sage: MS = MatrixSpace(ZZ, 2)
sage: MS.one().is_unit()
True
sage: MS.zero().is_unit()
False
sage: MS([1,2,3,4]).is_unit()
Traceback (most recent call last):
...
NotImplementedError

class Rings.HomCategory(category, name=None)

Initializes this HomCategory

INPUT:
• category – the category whose Homsets are the objects of this category.
• name – An optional name for this category.

EXAMPLES:

We need to skip one test, since the hierarchy of hom categories isn’t consistent yet:

sage: C = sage.categories.category.HomCategory(Rings()); C
Category of hom sets in Category of rings
sage: TestSuite(C).run(skip=['_test_category_graph'])

class Rings.ParentMethods
bracket(x, y)

Returns the Lie bracket $$[x, y] = x y - y x$$ of $$x$$ and $$y$$.

INPUT:

• x, y – elements of self

EXAMPLES:

sage: F = AlgebrasWithBasis(QQ).example()
sage: F
An example of an algebra with basis: the free algebra on the generators ('a', 'b', 'c') over Rational Field
sage: a,b,c = F.algebra_generators()
sage: F.bracket(a,b)
B[word: ab] - B[word: ba]


This measures the default of commutation between $$x$$ and $$y$$. $$F$$ endowed with the bracket operation is a Lie algebra; in particular, it satisfies Jacobi’s identity:

sage: F.bracket( F.bracket(a,b), c) + F.bracket(F.bracket(b,c),a) + F.bracket(F.bracket(c,a),b)
0

characteristic()

Return the characteristic of this ring.

EXAMPLES:

sage: QQ.characteristic()
0
sage: GF(19).characteristic()
19
sage: Integers(8).characteristic()
8
sage: Zp(5).characteristic()
0

ideal(*args, **kwds)

Create an ideal of this ring.

NOTE:

The code is copied from the base class Ring. This is because there are rings that do not inherit from that class, such as matrix algebras. See trac ticket #7797.

INPUT:

• An element or a list/tuple/sequence of elements.
• coerce (optional bool, default True): First coerce the elements into this ring.
• side, optional string, one of "twosided" (default), "left", "right": determines whether the resulting ideal is twosided, a left ideal or a right ideal.

EXAMPLE:

sage: MS = MatrixSpace(QQ,2,2)
sage: isinstance(MS,Ring)
False
sage: MS in Rings()
True
sage: MS.ideal(2)
Twosided Ideal
(
[2 0]
[0 2]
)
of Full MatrixSpace of 2 by 2 dense matrices over Rational Field
sage: MS.ideal([MS.0,MS.1],side='right')
Right Ideal
(
[1 0]
[0 0],

[0 1]
[0 0]
)
of Full MatrixSpace of 2 by 2 dense matrices over Rational Field

ideal_monoid()

The monoid of the ideals of this ring.

NOTE:

The code is copied from the base class of rings. This is since there are rings that do not inherit from that class, such as matrix algebras. See trac ticket #7797.

EXAMPLE:

sage: MS = MatrixSpace(QQ,2,2)
sage: isinstance(MS,Ring)
False
sage: MS in Rings()
True
sage: MS.ideal_monoid()
Monoid of ideals of Full MatrixSpace of 2 by 2 dense matrices
over Rational Field


Note that the monoid is cached:

sage: MS.ideal_monoid() is MS.ideal_monoid()
True

is_ring()

Return True, since this in an object of the category of rings.

EXAMPLES:

sage: Parent(QQ,category=Rings()).is_ring()
True

quo(I, names=None)

Quotient of a ring by a two-sided ideal.

NOTE:

This is a synonyme for quotient().

EXAMPLE:

sage: MS = MatrixSpace(QQ,2)
sage: MS.full_category_initialisation()
sage: I = MS*MS.gens()*MS


MS is not an instance of Ring. But since its category was fully initalised (which is not by default, by trac ticket #11900), it is an instance of the parent class of the category of rings. The quotient method is inherited from there:

sage: isinstance(MS,sage.rings.ring.Ring)
False
sage: isinstance(MS,Rings().parent_class)
True
sage: MS.quo(I,names = ['a','b','c','d'])
Quotient of Full MatrixSpace of 2 by 2 dense matrices over Rational Field by the ideal
(
[1 0]
[0 0],

[0 1]
[0 0],

[0 0]
[1 0],

[0 0]
[0 1]
)

quotient(I, names=None)

Quotient of a ring by a two-sided ideal.

INPUT:

• I: A twosided ideal of this ring.
• names: a list of strings to be used as names for the variables in the quotient ring.

EXAMPLES:

Usually, a ring inherits a method sage.rings.ring.Ring.quotient(). So, we need a bit of effort to make the following example work with the category framework:

sage: F.<x,y,z> = FreeAlgebra(QQ)
sage: from sage.rings.noncommutative_ideals import Ideal_nc
sage: class PowerIdeal(Ideal_nc):
...    def __init__(self, R, n):
...        self._power = n
...        self._power = n
...        Ideal_nc.__init__(self,R,[R.prod(m) for m in CartesianProduct(*[R.gens()]*n)])
...    def reduce(self,x):
...        R = self.ring()
...        return add([c*R(m) for c,m in x if len(m)<self._power],R(0))
...
sage: I = PowerIdeal(F,3)
sage: Q = Rings().parent_class.quotient(F,I); Q
Quotient of Free Algebra on 3 generators (x, y, z) over Rational Field by the ideal (x^3, x^2*y, x^2*z, x*y*x, x*y^2, x*y*z, x*z*x, x*z*y, x*z^2, y*x^2, y*x*y, y*x*z, y^2*x, y^3, y^2*z, y*z*x, y*z*y, y*z^2, z*x^2, z*x*y, z*x*z, z*y*x, z*y^2, z*y*z, z^2*x, z^2*y, z^3)
sage: Q.0
xbar
sage: Q.1
ybar
sage: Q.2
zbar
sage: Q.0*Q.1
xbar*ybar
sage: Q.0*Q.1*Q.0
0

quotient_ring(I, names=None)

Quotient of a ring by a two-sided ideal.

NOTE:

This is a synonyme for quotient().

EXAMPLE:

sage: MS = MatrixSpace(QQ,2)
sage: I = MS*MS.gens()*MS


MS is not an instance of Ring, but it is an instance of the parent class of the category of rings. The quotient method is inherited from there:

sage: isinstance(MS,sage.rings.ring.Ring)
False
sage: isinstance(MS,Rings().parent_class)
True
sage: MS.quotient_ring(I,names = ['a','b','c','d'])
Quotient of Full MatrixSpace of 2 by 2 dense matrices over Rational Field by the ideal
(
[1 0]
[0 0],

[0 1]
[0 0],

[0 0]
[1 0],

[0 0]
[0 1]
)

Rings.super_categories()

EXAMPLES:

sage: Rings().super_categories()
[Category of rngs, Category of semirings]


Ring ideals

Rngs