# Weighted homogeneous elements of free algebras, in letterplace implementation.¶

AUTHOR:

class sage.algebras.letterplace.free_algebra_element_letterplace.FreeAlgebraElement_letterplace

Weighted homogeneous elements of a free associative unital algebra (letterplace implementation)

EXAMPLES:

sage: F.<x,y,z> = FreeAlgebra(QQ, implementation='letterplace')
sage: x+y
x + y
sage: x*y !=y*x
True
sage: I = F*[x*y+y*z,x^2+x*y-y*x-y^2]*F
sage: (y^3).reduce(I)
y*y*y
sage: (y^3).normal_form(I)
y*y*z - y*z*y + y*z*z


Here is an example with nontrivial degree weights:

sage: F.<x,y,z> = FreeAlgebra(QQ, implementation='letterplace', degrees=[2,1,3])
sage: I = F*[x*y-y*x, x^2+2*y*z, (x*y)^2-z^2]*F
sage: x.degree()
2
sage: y.degree()
1
sage: z.degree()
3
sage: (x*y)^3
x*y*x*y*x*y
sage: ((x*y)^3).normal_form(I)
z*z*y*x
sage: ((x*y)^3).degree()
9

degree()

Return the degree of this element.

NOTE:

Generators may have a positive integral degree weight. All elements must be weighted homogeneous.

EXAMPLE:

sage: F.<x,y,z> = FreeAlgebra(QQ, implementation='letterplace')
sage: ((x+y+z)^3).degree()
3
sage: F.<x,y,z> = FreeAlgebra(QQ, implementation='letterplace', degrees=[2,1,3])
sage: ((x*y+z)^3).degree()
9

lc()

The leading coefficient of this free algebra element, as element of the base ring.

EXAMPLE:

sage: F.<x,y,z> = FreeAlgebra(QQ, implementation='letterplace')
sage: ((2*x+3*y-4*z)^2*(5*y+6*z)).lc()
20
sage: ((2*x+3*y-4*z)^2*(5*y+6*z)).lc().parent() is F.base()
True
sage: F.<x,y,z> = FreeAlgebra(QQ, implementation='letterplace', degrees=[2,1,3])
sage: ((2*x*y+z)^2).lc()
4

letterplace_polynomial()

Return the commutative polynomial that is used internally to represent this free algebra element.

EXAMPLE:

sage: F.<x,y,z> = FreeAlgebra(QQ, implementation='letterplace')
sage: ((x+y-z)^2).letterplace_polynomial()
x*x_1 + x*y_1 - x*z_1 + y*x_1 + y*y_1 - y*z_1 - z*x_1 - z*y_1 + z*z_1


If degree weights are used, the letterplace polynomial is homogenized by slack variables:

sage: F.<x,y,z> = FreeAlgebra(QQ, implementation='letterplace', degrees=[2,1,3])
sage: ((x*y+z)^2).letterplace_polynomial()
x*x__1*y_2*x_3*x__4*y_5 + x*x__1*y_2*z_3*x__4*x__5 + z*x__1*x__2*x_3*x__4*y_5 + z*x__1*x__2*z_3*x__4*x__5

lm()

The leading monomial of this free algebra element.

EXAMPLE:

sage: F.<x,y,z> = FreeAlgebra(QQ, implementation='letterplace')
sage: ((2*x+3*y-4*z)^2*(5*y+6*z)).lm()
x*x*y
sage: F.<x,y,z> = FreeAlgebra(QQ, implementation='letterplace', degrees=[2,1,3])
sage: ((2*x*y+z)^2).lm()
x*y*x*y

lm_divides(p)

Tell whether or not the leading monomial of self devides the leading monomial of another element.

NOTE:

A free algebra element $$p$$ divides another one $$q$$ if there are free algebra elements $$s$$ and $$t$$ such that $$spt = q$$.

EXAMPLE:

sage: F.<x,y,z> = FreeAlgebra(QQ, implementation='letterplace', degrees=[2,1,3])
sage: ((2*x*y+z)^2*z).lm()
x*y*x*y*z
sage: (y*x*y-y^4).lm()
y*x*y
sage: (y*x*y-y^4).lm_divides((2*x*y+z)^2*z)
True

lt()

The leading term (monomial times coefficient) of this free algebra element.

EXAMPLE:

sage: F.<x,y,z> = FreeAlgebra(QQ, implementation='letterplace')
sage: ((2*x+3*y-4*z)^2*(5*y+6*z)).lt()
20*x*x*y
sage: F.<x,y,z> = FreeAlgebra(QQ, implementation='letterplace', degrees=[2,1,3])
sage: ((2*x*y+z)^2).lt()
4*x*y*x*y

normal_form(I)

Return the normal form of this element with respect to a twosided weighted homogeneous ideal.

INPUT:

A twosided homogeneous ideal $$I$$ of the parent $$F$$ of this element, $$x$$.

OUTPUT:

The normal form of $$x$$ wrt. $$I$$.

NOTE:

The normal form is computed by reduction with respect to a Groebnerbasis of $$I$$ with degree bound $$deg(x)$$.

EXAMPLE:

sage: F.<x,y,z> = FreeAlgebra(QQ, implementation='letterplace')
sage: I = F*[x*y+y*z,x^2+x*y-y*x-y^2]*F
sage: (x^5).normal_form(I)
-y*z*z*z*x - y*z*z*z*y - y*z*z*z*z


We verify two basic properties of normal forms: The difference of an element and its normal form is contained in the ideal, and if two elements of the free algebra differ by an element of the ideal then they have the same normal form:

sage: x^5 - (x^5).normal_form(I) in I
True
sage: (x^5+x*I.0*y*z-3*z^2*I.1*y).normal_form(I) == (x^5).normal_form(I)
True


Here is an example with non-trivial degree weights:

sage: F.<x,y,z> = FreeAlgebra(QQ, implementation='letterplace', degrees=[1,2,3])
sage: I = F*[x*y-y*x+z, y^2+2*x*z, (x*y)^2-z^2]*F
sage: ((x*y)^3).normal_form(I)
z*z*y*x - z*z*z
sage: (x*y)^3-((x*y)^3).normal_form(I) in I
True
sage: ((x*y)^3+2*z*I.0*z+y*I.1*z-x*I.2*y).normal_form(I) == ((x*y)^3).normal_form(I)
True

reduce(G)

Reduce this element by a list of elements or by a twosided weighted homogeneous ideal.

INPUT:

Either a list or tuple of weighted homogeneous elements of the free algebra, or an ideal of the free algebra, or an ideal in the commutative polynomial ring that is currently used to implement the multiplication in the free algebra.

OUTPUT:

The twosided reduction of this element by the argument.

NOTE:

This may not be the normal form of this element, unless the argument is a twosided Groebner basis up to the degree of this element.

EXAMPLE:

sage: F.<x,y,z> = FreeAlgebra(QQ, implementation='letterplace')
sage: I = F*[x*y+y*z,x^2+x*y-y*x-y^2]*F
sage: p = y^2*z*y^2+y*z*y*z*y


We compute the letterplace version of the Groebneer basis of $$I$$ with degree bound 4:

sage: G = F._reductor_(I.groebner_basis(4).gens(),4)
sage: G.ring() is F.current_ring()
True


Since the element $$p$$ is of degree 5, it is no surrprise that its reductions with respect to the original generators of $$I$$ (of degree 2), or with respect to $$G$$ (Groebner basis with degree bound 4), or with respect to the Groebner basis with degree bound 5 (which yields its normal form) are pairwise different:

sage: p.reduce(I)
y*y*z*y*y + y*z*y*z*y
sage: p.reduce(G)
y*y*z*z*y + y*z*y*z*y - y*z*z*y*y + y*z*z*z*y
sage: p.normal_form(I)
y*y*z*z*z + y*z*y*z*z - y*z*z*y*z + y*z*z*z*z
sage: p.reduce(I) != p.reduce(G) != p.normal_form(I) != p.reduce(I)
True

sage.algebras.letterplace.free_algebra_element_letterplace.poly_reduce(ring=None, interruptible=True, attributes=None, *args)

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

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)

EXAMPLE:

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 ‘NF’ is given below.

5.1.119 reduce
--------------

*Syntax:*'
reduce (' poly_expression,' ideal_expression )'
reduce (' poly_expression,' ideal_expression,' int_expression
)'
reduce (' poly_expression,' poly_expression,' ideal_expression
)'
reduce (' vector_expression,' ideal_expression )'
reduce (' vector_expression,' ideal_expression,' int_expression
)'
reduce (' vector_expression,' module_expression )'
reduce (' vector_expression,' module_expression,'
int_expression )'
reduce (' vector_expression,' poly_expression,'
module_expression )'
reduce (' ideal_expression,' ideal_expression )'
reduce (' ideal_expression,' ideal_expression,' int_expression
)'
reduce (' ideal_expression,' matrix_expression,'
ideal_expression )'
reduce (' module_expression,' ideal_expression )'
reduce (' module_expression,' ideal_expression,' int_expression
)'
reduce (' module_expression,' module_expression )'
reduce (' module_expression,' module_expression,'
int_expression )'
reduce (' module_expression,' matrix_expression,'
module_expression )'
reduce (' poly/vector/ideal/module,' ideal/module,' int,'
intvec )'
reduce (' ideal,' matrix,' ideal,' int )'
reduce (' poly,' poly,' ideal,' int )'
reduce (' poly,' poly,' ideal,' int,' intvec )'

*Type:*'
the type of the first argument

*Purpose:*'
reduces a polynomial, vector, ideal  or module to its normal form
with respect to an ideal or module represented by a standard basis.
Returns 0 if and only if the polynomial (resp. vector, ideal,
module) is an element (resp. subideal, submodule) of the ideal
(resp. module).  The result may have no meaning if the second
argument is not a standard basis.
The third (optional) argument of type int modifies the behavior:
* 0 default

* 1 consider only the leading term and do no tail reduction.

* 2 reduce also with bad ecart (in the local case)

* 4 reduce without division, return possibly a non-zero
constant multiple of the remainder

If a second argument u' of type poly or matrix is given, the
first argument p' is replaced by p/u'.  This works only for zero
dimensional ideals (resp. modules) in the third argument and
gives, even in a local ring, a reduced normal form which is the
projection to the quotient by the ideal (resp. module).  One may
give a degree bound in the fourth argument with respect to a
weight vector in the fifth argument in order have a finite
computation.  If some of the weights are zero, the procedure may
not terminate!

*Note_*'
The commands reduce' and NF' are synonymous.

*Example:*'
ring r1 = 0,(z,y,x),ds;
poly s1=2x5y+7x2y4+3x2yz3;
poly s2=1x2y2z2+3z8;
poly s3=4xy5+2x2y2z3+11x10;
ideal i=s1,s2,s3;
ideal j=std(i);
reduce(3z3yx2+7y4x2+yx5+z12y2x2,j);
==> -yx5+2401/81y14x2+2744/81y11x5+392/27y8x8+224/81y5x11+16/81y2x14
reduce(3z3yx2+7y4x2+yx5+z12y2x2,j,1);
==> -yx5+z12y2x2
// 4 arguments:
ring rs=0,x,ds;
// normalform of 1/(1+x) w.r.t. (x3) up to degree 5
reduce(poly(1),1+x,ideal(x3),5);
==> // ** _ is no standard basis
==> 1-x+x2

See
* ideal::
* module::
* std::
* vector::

sage.algebras.letterplace.free_algebra_element_letterplace.singular_system(ring=None, interruptible=True, attributes=None, *args)

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

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)

EXAMPLE:

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 ‘system’ is given below.

5.1.141 system
--------------

*Syntax:*'
system (' string_expression )'
system (' string_expression,' expression )'

*Type:*'
depends on the desired function, may be none

*Purpose:*'
interface to internal data and the operating system. The
string_expression determines the command to execute. Some commands
require an additional argument (second form) where the type of the
argument depends on the command. See below for a list of all
possible commands.

*Note_*'
Not all functions work on every platform.

*Functions:*'

system("sh"', string_expression )'
shell escape, returns the return code of the shell as int.
The string is sent literally to the shell.

system("pid")'
returns the process number as int (for creating unique names).

system("--cpus")'
returns the number of cpu cores as int (for using multiple
cores).

system("uname")'
returns a string identifying the architecture for which
SINGULAR was compiled.

system("getenv",' string_expression)'
returns the value of the shell environment variable given as
the second argument. The return type is string.

system("setenv",'string_expression, string_expression)'
sets the shell environment variable given as the second
argument to the value given as the third argument. Returns
the third argument. Might not be available on all platforms.

system("tty")'
resets the terminal.

system("version")'
returns the version number of  SINGULAR as int.

system("contributors")'
returns names of people who contributed to the SINGULAR
kernel as string.

system("gen")'
returns the generating element of the multiplicative group of
(Z/p)\{0} (as int) where p is the characteristic of the
basering.

system("nblocks")'

system("nblocks",' ring_name )'
returns the number of blocks of the given ring, or the number
of parameters of the current basering, if no second argument
is given. The return type is int.

system("Singular")'
returns the absolute (path) name of the running SINGULAR as
string.

system("SingularLib")'
returns the colon seperated library search path name as
string.

system("'-")'
prints the values of all options.

system("'-long_option_name")'
returns the value of the (command-line) option
long_option_name. The type of the returned value is either

system("'-long_option_name",' expression)'
sets the value of the (command-line) option long_option_name
to the value given by the expression. Type of the expression
must be string, or int.  *Note Command line options::, for
seed of the random number generator, the used help browser,
the minimal display time, or the timer resolution.

system("browsers");'
returns a string about available help browsers.  *Note The

system("pid")'

*Example:*'
// a listing of the current directory:
system("sh","ls");
system("sh","sh");
string unique_name="/tmp/xx"+string(system("pid"));
unique_name;
==> /tmp/xx4711
system("uname")
==> ix86-Linux
system("getenv","PATH");
==> /bin:/usr/bin:/usr/local/bin
system("Singular");
==> /usr/local/bin/Singular
// report value of all options
system("--");
==> // --batch           0
==> // --execute
==> // --sdb             0
==> // --echo            1
==> // --quiet           1
==> // --sort            0
==> // --random          12345678
==> // --no-tty          1
==> // --user-option
==> // --allow-net       0
==> // --browser
==> // --cntrlc
==> // --emacs           0
==> // --no-stdlib       0
==> // --no-rc           1
==> // --no-warn         0
==> // --no-out          0
==> // --min-time        "0.5"
==> // --cpus            2
==> // --MPport
==> // --MPhost
==> // --MPrsh
==> // --ticks-per-sec   1
==> // --MPtransp
==> // --MPmode
// set minimal display time to 0.02 seconds
system("--min-time", "0.02");
// set timer resolution to 0.01 seconds
system("--ticks-per-sec", 100);
// re-seed random number generator
system("--random", 12345678);
system("--allow-net", 1);
// and set help browser to firefox
system("--browser", "firefox");
==> // ** Could not get IdxFile.
==> // ** Either set environment variable SINGULAR_IDX_FILE to IdxFile,
==> // ** or make sure that IdxFile is at /scratch/hannes/billbo-master/doc/s\
ingular.idx
==> // ** resource x not found
==> // ** Setting help browser to 'builtin'.
`

#### Previous topic

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

#### Next topic

Homogeneous ideals of free algebras.