Rings¶
- class sage.categories.rings.Rings(base_category)¶
Bases:
sage.categories.category_with_axiom.CategoryWithAxiom_singleton
The category of rings
Associative rings with unit, not necessarily commutative
EXAMPLES:
sage: Rings() Category of rings sage: sorted(Rings().super_categories(), key=str) [Category of rngs, Category of semirings] sage: sorted(Rings().axioms()) ['AdditiveAssociative', 'AdditiveCommutative', 'AdditiveInverse', 'AdditiveUnital', 'Associative', 'Distributive', 'Unital'] sage: Rings() is (CommutativeAdditiveGroups() & Monoids()).Distributive() True sage: Rings() is Rngs().Unital() True sage: Rings() is Semirings().AdditiveInverse() True
Todo
(see: http://trac.sagemath.org/sage_trac/wiki/CategoriesRoadMap)
Make Rings() into a subcategory or alias of Algebras(ZZ);
A parent P in the category
Rings()
should automatically be in the categoryAlgebras(P)
.
- Commutative¶
- Division¶
- class ElementMethods¶
Bases:
object
- inverse_of_unit()¶
Return the inverse of this element if it is a unit.
OUTPUT:
An element in the same ring as this element.
EXAMPLES:
sage: R.<x> = ZZ[] sage: S = R.quo(x^2 + x + 1) sage: S(1).inverse_of_unit() 1
This method fails when the element is not a unit:
sage: 2.inverse_of_unit() Traceback (most recent call last): ... ArithmeticError: inverse does not exist
The inverse returned is in the same ring as this element:
sage: a = -1 sage: a.parent() Integer Ring sage: a.inverse_of_unit().parent() Integer Ring
Note that this is often not the case when computing inverses in other ways:
sage: (~a).parent() Rational Field sage: (1/a).parent() Rational Field
- 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() False
- class MorphismMethods¶
Bases:
object
- extend_to_fraction_field()¶
Return the extension of this morphism to fraction fields of the domain and the codomain.
EXAMPLES:
sage: S.<x> = QQ[] sage: f = S.hom([x+1]); f Ring endomorphism of Univariate Polynomial Ring in x over Rational Field Defn: x |--> x + 1 sage: g = f.extend_to_fraction_field(); g Ring endomorphism of Fraction Field of Univariate Polynomial Ring in x over Rational Field Defn: x |--> x + 1 sage: g(x) x + 1 sage: g(1/x) 1/(x + 1)
If this morphism is not injective, it does not extend to the fraction field and an error is raised:
sage: f = GF(5).coerce_map_from(ZZ) sage: f.extend_to_fraction_field() Traceback (most recent call last): ... ValueError: the morphism is not injective
- is_injective()¶
Return whether or not this morphism is injective.
EXAMPLES:
sage: R.<x,y> = QQ[] sage: R.hom([x, y^2], R).is_injective() True sage: R.hom([x, x^2], R).is_injective() False sage: S.<u,v> = R.quotient(x^3*y) sage: R.hom([v, u], S).is_injective() False sage: S.hom([-u, v], S).is_injective() True sage: S.cover().is_injective() False
If the domain is a field, the homomorphism is injective:
sage: K.<x> = FunctionField(QQ) sage: L.<y> = FunctionField(QQ) sage: f = K.hom([y]); f Function Field morphism: From: Rational function field in x over Rational Field To: Rational function field in y over Rational Field Defn: x |--> y sage: f.is_injective() True
Unless the codomain is the zero ring:
sage: codomain = Integers(1) sage: f = QQ.hom([Zmod(1)(0)], check=False) sage: f.is_injective() False
Homomorphism from rings of characteristic zero to rings of positive characteristic can not be injective:
sage: R.<x> = ZZ[] sage: f = R.hom([GF(3)(1)]); f Ring morphism: From: Univariate Polynomial Ring in x over Integer Ring To: Finite Field of size 3 Defn: x |--> 1 sage: f.is_injective() False
A morphism whose domain is an order in a number field is injective if the codomain has characteristic zero:
sage: K.<x> = FunctionField(QQ) sage: f = ZZ.hom(K); f Composite map: From: Integer Ring To: Rational function field in x over Rational Field Defn: Conversion via FractionFieldElement_1poly_field map: From: Integer Ring To: Fraction Field of Univariate Polynomial Ring in x over Rational Field then Isomorphism: From: Fraction Field of Univariate Polynomial Ring in x over Rational Field To: Rational function field in x over Rational Field sage: f.is_injective() True
A coercion to the fraction field is injective:
sage: R = ZpFM(3) sage: R.fraction_field().coerce_map_from(R).is_injective() True
- NoZeroDivisors¶
alias of
sage.categories.domains.Domains
- class ParentMethods¶
Bases:
object
- bracket(x, y)¶
Returns the Lie bracket \([x, y] = x y - y x\) of \(x\) and \(y\).
INPUT:
x
,y
– elements ofself
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
- free_module(base=None, basis=None, map=True)¶
Return a free module \(V\) over the specified subring together with maps to and from \(V\).
The default implementation only supports the case that the base ring is the ring itself.
INPUT:
base
– a subring \(R\) so that this ring is isomorphic to a finite-rank free \(R\)-module \(V\)basis
– (optional) a basis for this ring over the basemap
– boolean (defaultTrue
), whether to return \(R\)-linear maps to and from \(V\)
OUTPUT:
A finite-rank free \(R\)-module \(V\)
An \(R\)-module isomorphism from \(V\) to this ring (only included if
map
isTrue
)An \(R\)-module isomorphism from this ring to \(V\) (only included if
map
isTrue
)
EXAMPLES:
sage: R.<x> = QQ[[]] sage: V, from_V, to_V = R.free_module(R) sage: v = to_V(1+x); v (1 + x) sage: from_V(v) 1 + x sage: W, from_W, to_W = R.free_module(R, basis=(1-x)) sage: W is V True sage: w = to_W(1+x); w (1 - x^2) sage: from_W(w) 1 + x + O(x^20)
- 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, defaultTrue
): 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.
EXAMPLES:
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.
EXAMPLES:
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
- is_zero()¶
Return
True
if this is the zero ring.EXAMPLES:
sage: Integers(1).is_zero() True sage: Integers(2).is_zero() False sage: QQ.is_zero() False sage: R.<x> = ZZ[] sage: R.quo(1).is_zero() True sage: R.<x> = GF(101)[] sage: R.quo(77).is_zero() True sage: R.quo(x^2+1).is_zero() False
- quo(I, names=None, **kwds)¶
Quotient of a ring by a two-sided ideal.
NOTE:
This is a synonym for
quotient()
.EXAMPLES:
sage: MS = MatrixSpace(QQ,2) sage: I = MS*MS.gens()*MS
MS
is not an instance ofRing
.However 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, **kwds)¶
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.further named arguments that may be passed to the quotient ring constructor.
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: from itertools import product sage: class PowerIdeal(Ideal_nc): ....: def __init__(self, R, n): ....: self._power = n ....: Ideal_nc.__init__(self, R, [R.prod(m) for m in product(R.gens(), repeat=n)]) ....: def reduce(self, x): ....: R = self.ring() ....: return add([c*R(m) for m,c 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, **kwds)¶
Quotient of a ring by a two-sided ideal.
NOTE:
This is a synonyme for
quotient()
.EXAMPLES:
sage: MS = MatrixSpace(QQ,2) sage: I = MS*MS.gens()*MS
MS
is not an instance ofRing
, 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] )
- class SubcategoryMethods¶
Bases:
object
- Division()¶
Return the full subcategory of the division objects of
self
.A ring satisfies the division axiom if all non-zero elements have multiplicative inverses.
Note
This could be generalized to
MagmasAndAdditiveMagmas.Distributive.AdditiveUnital
.EXAMPLES:
sage: Rings().Division() Category of division rings sage: Rings().Commutative().Division() Category of fields
- NoZeroDivisors()¶
Return the full subcategory of the objects of
self
having no nonzero zero divisors.A zero divisor in a ring \(R\) is an element \(x \in R\) such that there exists a nonzero element \(y \in R\) such that \(x \cdot y = 0\) or \(y \cdot x = 0\) (see Wikipedia article Zero_divisor).
EXAMPLES:
sage: Rings().NoZeroDivisors() Category of domains
Note
This could be generalized to
MagmasAndAdditiveMagmas.Distributive.AdditiveUnital
.