# Free associative unital algebras, implemented via Singular’s letterplace rings¶

AUTHOR:

With this implementation, Groebner bases out to a degree bound and normal forms can be computed for twosided weighted homogeneous ideals of free algebras. For now, all computations are restricted to weighted homogeneous elements, i.e., other elements cannot be created by arithmetic operations.

EXAMPLES:

sage: F.<x,y,z> = FreeAlgebra(QQ, implementation='letterplace')
sage: F
Free Associative Unital Algebra on 3 generators (x, y, z) over Rational Field
sage: I = F*[x*y+y*z,x^2+x*y-y*x-y^2]*F
sage: I
Twosided Ideal (x*y + y*z, x*x + x*y - y*x - y*y) of Free Associative Unital Algebra on 3 generators (x, y, z) over Rational Field
sage: x*(x*I.0-I.1*y+I.0*y)-I.1*y*z
x*y*x*y + x*y*y*y - x*y*y*z + x*y*z*y + y*x*y*z + y*y*y*z
sage: x^2*I.0-x*I.1*y+x*I.0*y-I.1*y*z in I
True


The preceding containment test is based on the computation of Groebner bases with degree bound:

sage: I.groebner_basis(degbound=4)
Twosided Ideal (x*y + y*z,
x*x - y*x - y*y - y*z,
y*y*y - y*y*z + y*z*y - y*z*z,
y*y*x + y*y*z + y*z*x + y*z*z,
y*y*z*y - y*y*z*z + y*z*z*y - y*z*z*z,
y*z*y*y - y*z*y*z + y*z*z*y - y*z*z*z,
y*y*z*x + y*y*z*z + y*z*z*x + y*z*z*z,
y*z*y*x + y*z*y*z + y*z*z*x + y*z*z*z) of Free Associative Unital
Algebra on 3 generators (x, y, z) over Rational Field


When reducing an element by $$I$$, the original generators are chosen:

sage: (y*z*y*y).reduce(I)
y*z*y*y


However, there is a method for computing the normal form of an element, which is the same as reduction by the Groebner basis out to the degree of that element:

sage: (y*z*y*y).normal_form(I)
y*z*y*z - y*z*z*y + y*z*z*z
sage: (y*z*y*y).reduce(I.groebner_basis(4))
y*z*y*z - y*z*z*y + y*z*z*z


The default term order derives from the degree reverse lexicographic order on the commutative version of the free algebra:

sage: F.commutative_ring().term_order()
Degree reverse lexicographic term order


A different term order can be chosen, and of course may yield a different normal form:

sage: L.<a,b,c> = FreeAlgebra(QQ, implementation='letterplace', order='lex')
sage: L.commutative_ring().term_order()
Lexicographic term order
sage: J = L*[a*b+b*c,a^2+a*b-b*c-c^2]*L
sage: J.groebner_basis(4)
Twosided Ideal (2*b*c*b - b*c*c + c*c*b,
a*b + b*c,
-a*c*c + 2*b*c*a + 2*b*c*c + c*c*a,
a*c*c*b - 2*b*c*c*b + b*c*c*c,
a*a - 2*b*c - c*c,
a*c*c*a - 2*b*c*c*a - 4*b*c*c*c - c*c*c*c) of Free Associative Unital
Algebra on 3 generators (a, b, c) over Rational Field
sage: (b*c*b*b).normal_form(J)
1/2*b*c*c*b - 1/2*c*c*b*b


Here is an example with degree weights:

sage: F.<x,y,z> = FreeAlgebra(QQ, implementation='letterplace', degrees=[1,2,3])
sage: (x*y+z).degree()
3


Todo

The computation of Groebner bases only works for global term orderings, and all elements must be weighted homogeneous with respect to positive integral degree weights. It is ongoing work in Singular to lift these restrictions.

We support coercion from the letterplace wrapper to the corresponding generic implementation of a free algebra (FreeAlgebra_generic), but there is no coercion in the opposite direction, since the generic implementation also comprises non-homogeneous elements.

We also do not support coercion from a subalgebra, or between free algebras with different term orderings, yet.

class sage.algebras.letterplace.free_algebra_letterplace.FreeAlgebra_letterplace

Finitely generated free algebra, with arithmetic restricted to weighted homogeneous elements.

Note

The restriction to weighted homogeneous elements should be lifted as soon as the restriction to homogeneous elements is lifted in Singular’s “Letterplace algebras”.

EXAMPLES:

sage: K.<z> = GF(25)
sage: F.<a,b,c> = FreeAlgebra(K, implementation='letterplace')
sage: F
Free Associative Unital Algebra on 3 generators (a, b, c) over Finite Field in z of size 5^2
sage: P = F.commutative_ring()
sage: P
Multivariate Polynomial Ring in a, b, c over Finite Field in z of size 5^2


We can do arithmetic as usual, as long as we stay (weighted) homogeneous:

sage: (z*a+(z+1)*b+2*c)^2
(z + 3)*a*a + (2*z + 3)*a*b + (2*z)*a*c + (2*z + 3)*b*a + (3*z + 4)*b*b + (2*z + 2)*b*c + (2*z)*c*a + (2*z + 2)*c*b - c*c
sage: a+1
Traceback (most recent call last):
...
ArithmeticError: can only add elements of the same weighted degree

commutative_ring()

Return the commutative version of this free algebra.

NOTE:

This commutative ring is used as a unique key of the free algebra.

EXAMPLES:

sage: K.<z> = GF(25)
sage: F.<a,b,c> = FreeAlgebra(K, implementation='letterplace')
sage: F
Free Associative Unital Algebra on 3 generators (a, b, c) over Finite Field in z of size 5^2
sage: F.commutative_ring()
Multivariate Polynomial Ring in a, b, c over Finite Field in z of size 5^2
sage: FreeAlgebra(F.commutative_ring()) is F
True

current_ring()

Return the commutative ring that is used to emulate the non-commutative multiplication out to the current degree.

EXAMPLES:

sage: F.<a,b,c> = FreeAlgebra(QQ, implementation='letterplace')
sage: F.current_ring()
Multivariate Polynomial Ring in a, b, c over Rational Field
sage: a*b
a*b
sage: F.current_ring()
Multivariate Polynomial Ring in a, b, c, a_1, b_1, c_1 over Rational Field
sage: F.set_degbound(3)
sage: F.current_ring()
Multivariate Polynomial Ring in a, b, c, a_1, b_1, c_1, a_2, b_2, c_2 over Rational Field

degbound()

Return the degree bound that is currently used.

NOTE:

When multiplying two elements of this free algebra, the degree bound will be dynamically adapted. It can also be set by set_degbound().

EXAMPLES:

In order to avoid we get a free algebras from the cache that was created in another doctest and has a different degree bound, we choose a base ring that does not appear in other tests:

sage: F.<x,y,z> = FreeAlgebra(ZZ, implementation='letterplace')
sage: F.degbound()
1
sage: x*y
x*y
sage: F.degbound()
2
sage: F.set_degbound(4)
sage: F.degbound()
4

gen(i)

Return the $$i$$-th generator.

INPUT:

$$i$$ – an integer.

OUTPUT:

Generator number $$i$$.

EXAMPLES:

sage: F.<a,b,c> = FreeAlgebra(QQ, implementation='letterplace')
sage: F.1 is F.1  # indirect doctest
True
sage: F.gen(2)
c

generator_degrees()
ideal_monoid()

Return the monoid of ideals of this free algebra.

EXAMPLES:

sage: F.<x,y> = FreeAlgebra(GF(2), implementation='letterplace')
sage: F.ideal_monoid()
Monoid of ideals of Free Associative Unital Algebra on 2 generators (x, y) over Finite Field of size 2
sage: F.ideal_monoid() is F.ideal_monoid()
True

is_commutative()

Tell whether this algebra is commutative, i.e., whether the generator number is one.

EXAMPLES:

sage: F.<x,y,z> = FreeAlgebra(QQ, implementation='letterplace')
sage: F.is_commutative()
False
sage: FreeAlgebra(QQ, implementation='letterplace', names=['x']).is_commutative()
True

is_field(proof=True)

Tell whether this free algebra is a field.

NOTE:

This would only be the case in the degenerate case of no generators. But such an example cannot be constructed in this implementation.

ngens()

Return the number of generators.

EXAMPLES:

sage: F.<a,b,c> = FreeAlgebra(QQ, implementation='letterplace')
sage: F.ngens()
3

set_degbound(d)

Increase the degree bound that is currently in place.

NOTE:

The degree bound cannot be decreased.

EXAMPLES:

In order to avoid we get a free algebras from the cache that was created in another doctest and has a different degree bound, we choose a base ring that does not appear in other tests:

sage: F.<x,y,z> = FreeAlgebra(GF(251), implementation='letterplace')
sage: F.degbound()
1
sage: x*y
x*y
sage: F.degbound()
2
sage: F.set_degbound(4)
sage: F.degbound()
4
sage: F.set_degbound(2)
sage: F.degbound()
4

term_order_of_block()

Return the term order that is used for the commutative version of this free algebra.

EXAMPLES:

sage: F.<x,y,z> = FreeAlgebra(QQ, implementation='letterplace')
sage: F.term_order_of_block()
Degree reverse lexicographic term order
sage: L.<a,b,c> = FreeAlgebra(QQ, implementation='letterplace',order='lex')
sage: L.term_order_of_block()
Lexicographic term order

class sage.algebras.letterplace.free_algebra_letterplace.FreeAlgebra_letterplace_libsingular

Bases: object

Internally used wrapper around a Singular Letterplace polynomial ring.

sage.algebras.letterplace.free_algebra_letterplace.freeAlgebra(ring=None, interruptible=True, attributes=None, *args)

This function is an automatically generated C wrapper around the Singular function ‘freeAlgebra’.

This wrapper takes care of converting Sage datatypes to Singular datatypes and vice versa. In addition to whatever parameters the underlying Singular function accepts when called, this function also accepts the following keyword parameters:

INPUT:

• args – a list of arguments

• ring – a multivariate polynomial ring

• interruptible – if True pressing Ctrl-C during the execution of this function will interrupt the computation (default: True)

• attributes – a dictionary of optional Singular attributes assigned to Singular objects (default: None)

If ring is not specified, it is guessed from the given arguments. If this is not possible, then a dummy ring, univariate polynomial ring over QQ, is used.

EXAMPLES:

sage: groebner = sage.libs.singular.function_factory.ff.groebner
sage: P.<x, y> = PolynomialRing(QQ)
sage: I = P.ideal(x^2-y, y+x)
sage: groebner(I)
[x + y, y^2 - y]
sage: triangL = sage.libs.singular.function_factory.ff.triang__lib.triangL
sage: P.<x1, x2> = PolynomialRing(QQ, order='lex')
sage: f1 = 1/2*((x1^2 + 2*x1 - 4)*x2^2 + 2*(x1^2 + x1)*x2 + x1^2)
sage: f2 = 1/2*((x1^2 + 2*x1 + 1)*x2^2 + 2*(x1^2 + x1)*x2 - 4*x1^2)
sage: I = Ideal(Ideal(f1,f2).groebner_basis()[::-1])
sage: triangL(I, attributes={I:{'isSB':1}})
[[x2^4 + 4*x2^3 - 6*x2^2 - 20*x2 + 5, 8*x1 - x2^3 + x2^2 + 13*x2 - 5],
[x2, x1^2],
[x2, x1^2],
[x2, x1^2]]


The Singular documentation for ‘freeAlgebra’ is given below.

Singular documentation not found