Quotient Rings¶
AUTHORS:
 William Stein
 Simon King (201104): Put it into the category framework, use the new coercion model.
 Simon King (201104): Quotients of noncommutative rings by twosided ideals.
Todo
The following skipped tests should be removed once trac ticket #13999 is fixed:
sage: TestSuite(S).run(skip=['_test_nonzero_equal', '_test_elements', '_test_zero'])
In trac ticket #11068, noncommutative quotient rings \(R/I\) were
implemented. The only requirement is that the twosided ideal \(I\)
provides a reduce
method so that I.reduce(x)
is the normal
form of an element \(x\) with respect to \(I\) (i.e., we have
I.reduce(x) == I.reduce(y)
if \(xy \in I\), and
x  I.reduce(x) in I
). Here is a toy example:
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
....: 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: F.<x,y,z> = FreeAlgebra(QQ, 3)
sage: I3 = PowerIdeal(F,3); I3
Twosided 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) of
Free Algebra on 3 generators (x, y, z) over Rational Field
Free algebras have a custom quotient method that serves at creating finite dimensional quotients defined by multiplication matrices. We are bypassing it, so that we obtain the default quotient:
sage: Q3.<a,b,c> = F.quotient(I3)
sage: Q3
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: (a+b+2)^4
16 + 32*a + 32*b + 24*a^2 + 24*a*b + 24*b*a + 24*b^2
sage: Q3.is_commutative()
False
Even though \(Q_3\) is not commutative, there is commutativity for products of degree three:
sage: a*(b*c)(b*c)*a==F.zero()
True
If we quotient out all terms of degree two then of course the resulting quotient ring is commutative:
sage: I2 = PowerIdeal(F,2); I2
Twosided Ideal (x^2, x*y, x*z, y*x, y^2, y*z, z*x, z*y, z^2) of Free Algebra
on 3 generators (x, y, z) over Rational Field
sage: Q2.<a,b,c> = F.quotient(I2)
sage: Q2.is_commutative()
True
sage: (a+b+2)^4
16 + 32*a + 32*b
Since trac ticket #7797, there is an implementation of free algebras based on Singular’s implementation of the Letterplace Algebra. Our letterplace wrapper allows to provide the above toy example more easily:
sage: from itertools import product
sage: F.<x,y,z> = FreeAlgebra(QQ, implementation='letterplace')
sage: Q3 = F.quo(F*[F.prod(m) for m in product(F.gens(), repeat=3)]*F)
sage: Q3
Quotient of Free Associative Unital Algebra on 3 generators (x, y, z) over Rational Field by the ideal (x*x*x, x*x*y, x*x*z, x*y*x, x*y*y, x*y*z, x*z*x, x*z*y, x*z*z, y*x*x, y*x*y, y*x*z, y*y*x, y*y*y, y*y*z, y*z*x, y*z*y, y*z*z, z*x*x, z*x*y, z*x*z, z*y*x, z*y*y, z*y*z, z*z*x, z*z*y, z*z*z)
sage: Q3.0*Q3.1Q3.1*Q3.0
xbar*ybar  ybar*xbar
sage: Q3.0*(Q3.1*Q3.2)(Q3.1*Q3.2)*Q3.0
0
sage: Q2 = F.quo(F*[F.prod(m) for m in product(F.gens(), repeat=2)]*F)
sage: Q2.is_commutative()
True

sage.rings.quotient_ring.
QuotientRing
(R, I, names=None)¶ Creates a quotient ring of the ring \(R\) by the twosided ideal \(I\).
Variables are labeled by
names
(if the quotient ring is a quotient of a polynomial ring). Ifnames
isn’t given, ‘bar’ will be appended to the variable names in \(R\).INPUT:
R
– a ring.I
– a twosided ideal of \(R\).names
– (optional) a list of strings to be used as names for the variables in the quotient ring \(R/I\).
OUTPUT: \(R/I\)  the quotient ring \(R\) mod the ideal \(I\)
ASSUMPTION:
I
has a methodI.reduce(x)
returning the normal form of elements \(x\in R\). In other words, it is required thatI.reduce(x)==I.reduce(y)
\(\iff xy \in I\), andxI.reduce(x) in I
, for all \(x,y\in R\).EXAMPLES:
Some simple quotient rings with the integers:
sage: R = QuotientRing(ZZ,7*ZZ); R Quotient of Integer Ring by the ideal (7) sage: R.gens() (1,) sage: 1*R(3); 6*R(3); 7*R(3) 3 4 0
sage: S = QuotientRing(ZZ,ZZ.ideal(8)); S Quotient of Integer Ring by the ideal (8) sage: 2*S(4) 0
With polynomial rings (note that the variable name of the quotient ring can be specified as shown below):
sage: R.<xx> = QuotientRing(QQ[x], QQ[x].ideal(x^2 + 1)); R Univariate Quotient Polynomial Ring in xx over Rational Field with modulus x^2 + 1 sage: R.gens(); R.gen() (xx,) xx sage: for n in range(4): xx^n 1 xx 1 xx
sage: S = QuotientRing(QQ[x], QQ[x].ideal(x^2  2)); S Univariate Quotient Polynomial Ring in xbar over Rational Field with modulus x^2  2 sage: xbar = S.gen(); S.gen() xbar sage: for n in range(3): xbar^n 1 xbar 2
Sage coerces objects into ideals when possible:
sage: R = QuotientRing(QQ[x], x^2 + 1); R Univariate Quotient Polynomial Ring in xbar over Rational Field with modulus x^2 + 1
By Noether’s homomorphism theorems, the quotient of a quotient ring of \(R\) is just the quotient of \(R\) by the sum of the ideals. In this example, we end up modding out the ideal \((x)\) from the ring \(\QQ[x,y]\):
sage: R.<x,y> = PolynomialRing(QQ,2) sage: S.<a,b> = QuotientRing(R,R.ideal(1 + y^2)) sage: T.<c,d> = QuotientRing(S,S.ideal(a)) sage: T Quotient of Multivariate Polynomial Ring in x, y over Rational Field by the ideal (x, y^2 + 1) sage: R.gens(); S.gens(); T.gens() (x, y) (a, b) (0, d) sage: for n in range(4): d^n 1 d 1 d

class
sage.rings.quotient_ring.
QuotientRing_generic
(R, I, names, category=None)¶ Bases:
sage.rings.quotient_ring.QuotientRing_nc
,sage.rings.ring.CommutativeRing
Creates a quotient ring of a commutative ring \(R\) by the ideal \(I\).
EXAMPLES:
sage: R.<x> = PolynomialRing(ZZ) sage: I = R.ideal([4 + 3*x + x^2, 1 + x^2]) sage: S = R.quotient_ring(I); S Quotient of Univariate Polynomial Ring in x over Integer Ring by the ideal (x^2 + 3*x + 4, x^2 + 1)

class
sage.rings.quotient_ring.
QuotientRing_nc
(R, I, names, category=None)¶ Bases:
sage.rings.ring.Ring
,sage.structure.parent_gens.ParentWithGens
The quotient ring of \(R\) by a twosided ideal \(I\).
This class is for rings that do not inherit from
CommutativeRing
.EXAMPLES:
Here is a quotient of a free algebra by a twosided homogeneous ideal:
sage: F.<x,y,z> = FreeAlgebra(QQ, implementation='letterplace') sage: I = F*[x*y+y*z,x^2+x*yy*xy^2]*F sage: Q.<a,b,c> = F.quo(I); Q Quotient of Free Associative Unital Algebra on 3 generators (x, y, z) over Rational Field by the ideal (x*y + y*z, x*x + x*y  y*x  y*y) sage: a*b b*c sage: a^3 b*c*a  b*c*b  b*c*c
A quotient of a quotient is just the quotient of the original top ring by the sum of two ideals:
sage: J = Q*[a^3b^3]*Q sage: R.<i,j,k> = Q.quo(J); R Quotient of Free Associative Unital Algebra on 3 generators (x, y, z) over Rational Field by the ideal (y*y*z  y*z*x  2*y*z*z, x*y + y*z, x*x + x*y  y*x  y*y) sage: i^3 j*k*i  j*k*j  j*k*k sage: j^3 j*k*i  j*k*j  j*k*k
For rings that do inherit from
CommutativeRing
, we provide a subclassQuotientRing_generic
, for backwards compatibility.EXAMPLES:
sage: R.<x> = PolynomialRing(ZZ,'x') sage: I = R.ideal([4 + 3*x + x^2, 1 + x^2]) sage: S = R.quotient_ring(I); S Quotient of Univariate Polynomial Ring in x over Integer Ring by the ideal (x^2 + 3*x + 4, x^2 + 1)
sage: R.<x,y> = PolynomialRing(QQ) sage: S.<a,b> = R.quo(x^2 + y^2) sage: a^2 + b^2 == 0 True sage: S(0) == a^2 + b^2 True
Again, a quotient of a quotient is just the quotient of the original top ring by the sum of two ideals.
sage: R.<x,y> = PolynomialRing(QQ,2) sage: S.<a,b> = R.quo(1 + y^2) sage: T.<c,d> = S.quo(a) sage: T Quotient of Multivariate Polynomial Ring in x, y over Rational Field by the ideal (x, y^2 + 1) sage: T.gens() (0, d)

Element
¶ alias of
QuotientRingElement

ambient
()¶ Returns the cover ring of the quotient ring: that is, the original ring \(R\) from which we modded out an ideal, \(I\).
EXAMPLES:
sage: Q = QuotientRing(ZZ,7*ZZ) sage: Q.cover_ring() Integer Ring
sage: Q = QuotientRing(QQ[x], x^2 + 1) sage: Q.cover_ring() Univariate Polynomial Ring in x over Rational Field

characteristic
()¶ Return the characteristic of the quotient ring.
Todo
Not yet implemented!
EXAMPLES:
sage: Q = QuotientRing(ZZ,7*ZZ) sage: Q.characteristic() Traceback (most recent call last): ... NotImplementedError

construction
()¶ Returns the functorial construction of
self
.EXAMPLES:
sage: R.<x> = PolynomialRing(ZZ,'x') sage: I = R.ideal([4 + 3*x + x^2, 1 + x^2]) sage: R.quotient_ring(I).construction() (QuotientFunctor, Univariate Polynomial Ring in x over Integer Ring) sage: F.<x,y,z> = FreeAlgebra(QQ, implementation='letterplace') sage: I = F*[x*y+y*z,x^2+x*yy*xy^2]*F sage: Q = F.quo(I) sage: Q.construction() (QuotientFunctor, Free Associative Unital Algebra on 3 generators (x, y, z) over Rational Field)

cover
()¶ The covering ring homomorphism \(R \to R/I\), equipped with a section.
EXAMPLES:
sage: R = ZZ.quo(3*ZZ) sage: pi = R.cover() sage: pi Ring morphism: From: Integer Ring To: Ring of integers modulo 3 Defn: Natural quotient map sage: pi(5) 2 sage: l = pi.lift()
sage: R.<x,y> = PolynomialRing(QQ) sage: Q = R.quo( (x^2,y^2) ) sage: pi = Q.cover() sage: pi(x^3+y) ybar sage: l = pi.lift(x+y^3) sage: l x sage: l = pi.lift(); l Settheoretic ring morphism: From: Quotient of Multivariate Polynomial Ring in x, y over Rational Field by the ideal (x^2, y^2) To: Multivariate Polynomial Ring in x, y over Rational Field Defn: Choice of lifting map sage: l(x+y^3) x

cover_ring
()¶ Returns the cover ring of the quotient ring: that is, the original ring \(R\) from which we modded out an ideal, \(I\).
EXAMPLES:
sage: Q = QuotientRing(ZZ,7*ZZ) sage: Q.cover_ring() Integer Ring
sage: Q = QuotientRing(QQ[x], x^2 + 1) sage: Q.cover_ring() Univariate Polynomial Ring in x over Rational Field

defining_ideal
()¶ Returns the ideal generating this quotient ring.
EXAMPLES:
In the integers:
sage: Q = QuotientRing(ZZ,7*ZZ) sage: Q.defining_ideal() Principal ideal (7) of Integer Ring
An example involving a quotient of a quotient. By Noether’s homomorphism theorems, this is actually a quotient by a sum of two ideals:
sage: R.<x,y> = PolynomialRing(QQ,2) sage: S.<a,b> = QuotientRing(R,R.ideal(1 + y^2)) sage: T.<c,d> = QuotientRing(S,S.ideal(a)) sage: S.defining_ideal() Ideal (y^2 + 1) of Multivariate Polynomial Ring in x, y over Rational Field sage: T.defining_ideal() Ideal (x, y^2 + 1) of Multivariate Polynomial Ring in x, y over Rational Field

gen
(i=0)¶ Returns the \(i\)th generator for this quotient ring.
EXAMPLES:
sage: R = QuotientRing(ZZ,7*ZZ) sage: R.gen(0) 1
sage: R.<x,y> = PolynomialRing(QQ,2) sage: S.<a,b> = QuotientRing(R,R.ideal(1 + y^2)) sage: T.<c,d> = QuotientRing(S,S.ideal(a)) sage: T Quotient of Multivariate Polynomial Ring in x, y over Rational Field by the ideal (x, y^2 + 1) sage: R.gen(0); R.gen(1) x y sage: S.gen(0); S.gen(1) a b sage: T.gen(0); T.gen(1) 0 d

ideal
(*gens, **kwds)¶ Return the ideal of
self
with the given generators.EXAMPLES:
sage: R.<x,y> = PolynomialRing(QQ) sage: S = R.quotient_ring(x^2+y^2) sage: S.ideal() Ideal (0) of Quotient of Multivariate Polynomial Ring in x, y over Rational Field by the ideal (x^2 + y^2) sage: S.ideal(x+y+1) Ideal (xbar + ybar + 1) of Quotient of Multivariate Polynomial Ring in x, y over Rational Field by the ideal (x^2 + y^2)

is_commutative
()¶ Tell whether this quotient ring is commutative.
Note
This is certainly the case if the cover ring is commutative. Otherwise, if this ring has a finite number of generators, it is tested whether they commute. If the number of generators is infinite, a
NotImplementedError
is raised.AUTHOR:
 Simon King (20110323): See trac ticket #7797.
EXAMPLES:
Any quotient of a commutative ring is commutative:
sage: P.<a,b,c> = QQ[] sage: P.quo(P.random_element()).is_commutative() True
The noncommutative case is more interesting:
sage: F.<x,y,z> = FreeAlgebra(QQ, implementation='letterplace') sage: I = F*[x*y+y*z,x^2+x*yy*xy^2]*F sage: Q = F.quo(I) sage: Q.is_commutative() False sage: Q.1*Q.2==Q.2*Q.1 False
In the next example, the generators apparently commute:
sage: J = F*[x*yy*x,x*zz*x,y*zz*y,x^3y^3]*F sage: R = F.quo(J) sage: R.is_commutative() True

is_field
(proof=True)¶ Returns
True
if the quotient ring is a field. Checks to see if the defining ideal is maximal.

is_integral_domain
(proof=True)¶ With
proof
equal toTrue
(the default), this function may raise aNotImplementedError
.When
proof
isFalse
, ifTrue
is returned, then self is definitely an integral domain. If the function returnsFalse
, then eitherself
is not an integral domain or it was unable to determine whether or notself
is an integral domain.EXAMPLES:
sage: R.<x,y> = QQ[] sage: R.quo(x^2  y).is_integral_domain() True sage: R.quo(x^2  y^2).is_integral_domain() False sage: R.quo(x^2  y^2).is_integral_domain(proof=False) False sage: R.<a,b,c> = ZZ[] sage: Q = R.quotient_ring([a, b]) sage: Q.is_integral_domain() Traceback (most recent call last): ... NotImplementedError sage: Q.is_integral_domain(proof=False) False

is_noetherian
()¶ Return
True
if this ring is Noetherian.EXAMPLES:
sage: R = QuotientRing(ZZ, 102*ZZ) sage: R.is_noetherian() True sage: R = QuotientRing(QQ[x], x^2+1) sage: R.is_noetherian() True
If the cover ring of
self
is not Noetherian, we currently have no way of testing whetherself
is Noetherian, so we raise an error:sage: R.<x> = InfinitePolynomialRing(QQ) sage: R.is_noetherian() False sage: I = R.ideal([x[1]^2, x[2]]) sage: S = R.quotient(I) sage: S.is_noetherian() Traceback (most recent call last): ... NotImplementedError

lift
(x=None)¶ Return the lifting map to the cover, or the image of an element under the lifting map.
Note
The category framework imposes that
Q.lift(x)
returns the image of an element \(x\) under the lifting map. For backwards compatibility, we letQ.lift()
return the lifting map.EXAMPLES:
sage: R.<x,y> = PolynomialRing(QQ, 2) sage: S = R.quotient(x^2 + y^2) sage: S.lift() Settheoretic ring morphism: From: Quotient of Multivariate Polynomial Ring in x, y over Rational Field by the ideal (x^2 + y^2) To: Multivariate Polynomial Ring in x, y over Rational Field Defn: Choice of lifting map sage: S.lift(S.0) == x True

lifting_map
()¶ Return the lifting map to the cover.
EXAMPLES:
sage: R.<x,y> = PolynomialRing(QQ, 2) sage: S = R.quotient(x^2 + y^2) sage: pi = S.cover(); pi Ring morphism: From: Multivariate Polynomial Ring in x, y over Rational Field To: Quotient of Multivariate Polynomial Ring in x, y over Rational Field by the ideal (x^2 + y^2) Defn: Natural quotient map sage: L = S.lifting_map(); L Settheoretic ring morphism: From: Quotient of Multivariate Polynomial Ring in x, y over Rational Field by the ideal (x^2 + y^2) To: Multivariate Polynomial Ring in x, y over Rational Field Defn: Choice of lifting map sage: L(S.0) x sage: L(S.1) y
Note that some reduction may be applied so that the lift of a reduction need not equal the original element:
sage: z = pi(x^3 + 2*y^2); z xbar*ybar^2 + 2*ybar^2 sage: L(z) x*y^2 + 2*y^2 sage: L(z) == x^3 + 2*y^2 False
Test that there also is a lift for rings that are no instances of
Ring
(see trac ticket #11068):sage: MS = MatrixSpace(GF(5),2,2) sage: I = MS*[MS.0*MS.1,MS.2+MS.3]*MS sage: Q = MS.quo(I) sage: Q.lift() Settheoretic ring morphism: From: Quotient of Full MatrixSpace of 2 by 2 dense matrices over Finite Field of size 5 by the ideal ( [0 1] [0 0], [0 0] [1 1] ) To: Full MatrixSpace of 2 by 2 dense matrices over Finite Field of size 5 Defn: Choice of lifting map

ngens
()¶ Returns the number of generators for this quotient ring.
Todo
Note that
ngens
counts 0 as a generator. Does this make sense? That is, since 0 only generates itself and the fact that this is true for all rings, is there a way to “knock it off” of the generators list if a generator of some original ring is modded out?EXAMPLES:
sage: R = QuotientRing(ZZ,7*ZZ) sage: R.gens(); R.ngens() (1,) 1
sage: R.<x,y> = PolynomialRing(QQ,2) sage: S.<a,b> = QuotientRing(R,R.ideal(1 + y^2)) sage: T.<c,d> = QuotientRing(S,S.ideal(a)) sage: T Quotient of Multivariate Polynomial Ring in x, y over Rational Field by the ideal (x, y^2 + 1) sage: R.gens(); S.gens(); T.gens() (x, y) (a, b) (0, d) sage: R.ngens(); S.ngens(); T.ngens() 2 2 2

retract
(x)¶ The image of an element of the cover ring under the quotient map.
INPUT:
x
– An element of the cover ring
OUTPUT:
The image of the given element in
self
.EXAMPLES:
sage: R.<x,y> = PolynomialRing(QQ, 2) sage: S = R.quotient(x^2 + y^2) sage: S.retract((x+y)^2) 2*xbar*ybar

term_order
()¶ Return the term order of this ring.
EXAMPLES:
sage: P.<a,b,c> = PolynomialRing(QQ) sage: I = Ideal([a^2  a, b^2  b, c^2  c]) sage: Q = P.quotient(I) sage: Q.term_order() Degree reverse lexicographic term order


sage.rings.quotient_ring.
is_QuotientRing
(x)¶ Tests whether or not
x
inherits fromQuotientRing_nc
.EXAMPLES:
sage: from sage.rings.quotient_ring import is_QuotientRing sage: R.<x> = PolynomialRing(ZZ,'x') sage: I = R.ideal([4 + 3*x + x^2, 1 + x^2]) sage: S = R.quotient_ring(I) sage: is_QuotientRing(S) True sage: is_QuotientRing(R) False
sage: F.<x,y,z> = FreeAlgebra(QQ, implementation='letterplace') sage: I = F*[x*y+y*z,x^2+x*yy*xy^2]*F sage: Q = F.quo(I) sage: is_QuotientRing(Q) True sage: is_QuotientRing(F) False