There is are some domains in FriCAS for doing computations with non-commuting variables developed by Michel Petitot. You can find some examples in the FriCAS book under the title XPolynomial? but unfortunately the explanations are a little terse. You can also check for documentation in the source files:

• src/algebra/xpoly.spad
• src/algebra/xlpoly.spad

and HyperDoc example page about XPOLY

In FriCAS algebra is simplified and there is no longer OrderedFreeMonoid?, we just use FreeMonoid below.

We need left and right quotients which are provided by divide to implement the substitution rules in the example below.

Test left and right exact quotients.

(1) -> m1:=(x*y*y*z)$FMONOID(Symbol)

(1)

m2:=(x*y)$FMONOID(Symbol)

(2)

lquo(m1,m2)

(3)

m3:=(y*y)$FMONOID(Symbol)

(4)

divide(m1,m2)

(5)

divide(m1,m3)

(6)

m4:=(y^3)$FMONOID(Symbol)

(7)

divide(m1,m4)

(8)

This option is required to compile the functions that follow.

)set function compile on

On Tuesday, February 28, 2006 6:54 AM Fabio S. wrote:

 I would like to build the non-commutative algebra h=k[x,y] and
 then I would like to make computations in h using some predefined 
 rules for x and y. As an example, take the three equations

 x*y*x=y*x*y
 x*x=a*x+b
 y*y=a*y+b

 where a and b are (generic, if possible) elements of k.

 Then, I would like to be able to reduce polynomials in x and 
 y according to the previous rules. For example,

 (x+y)^2 (=x^2+x*y+y*x+y^2)

 should reduce to

 a*(x+y)+2*b+x*y+y*x

--Generic elements of k
--OVAR = OrderedVariableList
C==>OVAR [a,b]

--Commutative Field: k=Q[a,b]
--Q = FRAC INT = Fration Integer
--SMP = SparseMultivariatePolynomials
K==>SMP(FRAC INT,C)

--Non-commutative variables
V==>OVAR [x,y]

--Non-commutative Algebra: h=k[x,y]
--XDPOLY XDistributedPolynomial
H==>XDPOLY(V,K)

--Free monoid
M==>FMONOID V

--Record giving result of division
Rec==>Record(lm : M, rm : M)

--Substitution rules are applied to words from the monoid over
--the variables and return polynomials
subs(w:M):H ==
  --x*y*x=y*x*y
  n:=divide(w,(x::V*y::V*x::V)$M)$M
  n case Rec => monomial(1, (n::Rec).lm)$H * (y::V*x::V*y::V)$H * monomial(1, (n::Rec).rm)$H
  --x*x=a*x+b
  n:=divide(w,(x::V^2)$M)$M
  n case Rec => monomial(1, (n::Rec).lm)$H * (a::K*x::V+b::K)$H * monomial(1, (n::Rec).rm)$H
  --y*y=a*y+b
  n:=divide(w,(y::V^2)$M)$M
  n case Rec => monomial(1, (n::Rec).lm)$H * (a::K*y::V+b::K)$H * monomial(1, (n::Rec).rm)$H
  --no change
  monomial(1, w)$H

   Function declaration subs : FreeMonoid(OrderedVariableList([x,y]))
       -> XDistributedPolynomial(OrderedVariableList([x,y]),
      SparseMultivariatePolynomial(Fraction(Integer),
      OrderedVariableList([a,b]))) has been added to workspace.

--Apply rules to a term. Keep coefficients 
newterm(x:Record(k:M,c:K)):H==x.c*subs(x.k)

   Function declaration newterm : Record(k: FreeMonoid(
      OrderedVariableList([x,y])),c: SparseMultivariatePolynomial(
      Fraction(Integer),OrderedVariableList([a,b]))) -> 
      XDistributedPolynomial(OrderedVariableList([x,y]),
      SparseMultivariatePolynomial(Fraction(Integer),
      OrderedVariableList([a,b]))) has been added to workspace.

--Reconstruct polynomial, term-by-term
newpoly(t:H):H==reduce(+,map(newterm,listOfTerms(t)))

   Function declaration newpoly : XDistributedPolynomial(
      OrderedVariableList([x,y]),SparseMultivariatePolynomial(Fraction(
      Integer),OrderedVariableList([a,b]))) -> XDistributedPolynomial(
      OrderedVariableList([x,y]),SparseMultivariatePolynomial(Fraction(
      Integer),OrderedVariableList([a,b]))) has been added to 
      workspace.

Example calculations:

p1:=(x::V+y::V)$H^2

(9)

newpoly(p1)

Compiling function newpoly with type XDistributedPolynomial(
      OrderedVariableList([x,y]),SparseMultivariatePolynomial(Fraction(
      Integer),OrderedVariableList([a,b]))) -> XDistributedPolynomial(
      OrderedVariableList([x,y]),SparseMultivariatePolynomial(Fraction(
      Integer),OrderedVariableList([a,b])))

Compiling function subs with type FreeMonoid(OrderedVariableList([x,
      y])) -> XDistributedPolynomial(OrderedVariableList([x,y]),
      SparseMultivariatePolynomial(Fraction(Integer),
      OrderedVariableList([a,b])))

Compiling function newterm with type Record(k: FreeMonoid(
      OrderedVariableList([x,y])),c: SparseMultivariatePolynomial(
      Fraction(Integer),OrderedVariableList([a,b]))) -> 
      XDistributedPolynomial(OrderedVariableList([x,y]),
      SparseMultivariatePolynomial(Fraction(Integer),
      OrderedVariableList([a,b])))

(10)

p2:=(x::V+y::V)$H^3

(11)

newpoly(p2)

(12)

newpoly above applies rules once. However the rules above should be applied more than once - they should be applied until no more changes are possible. This is done below:

reduce(p:H):H ==
  p2 := newpoly(p)
  p3 := newpoly(p2)
  while p3 ~= p2 repeat
   p2 := p3
   p3 := newpoly(p2)
  p3

   Function declaration reduce : XDistributedPolynomial(
      OrderedVariableList([x,y]),SparseMultivariatePolynomial(Fraction(
      Integer),OrderedVariableList([a,b]))) -> XDistributedPolynomial(
      OrderedVariableList([x,y]),SparseMultivariatePolynomial(Fraction(
      Integer),OrderedVariableList([a,b]))) has been added to 
      workspace.
   Compiled code for newpoly has been cleared.

reduce(p2)

Compiling function newpoly with type XDistributedPolynomial(
      OrderedVariableList([x,y]),SparseMultivariatePolynomial(Fraction(
      Integer),OrderedVariableList([a,b]))) -> XDistributedPolynomial(
      OrderedVariableList([x,y]),SparseMultivariatePolynomial(Fraction(
      Integer),OrderedVariableList([a,b])))

Compiling function reduce with type XDistributedPolynomial(
      OrderedVariableList([x,y]),SparseMultivariatePolynomial(Fraction(
      Integer),OrderedVariableList([a,b]))) -> XDistributedPolynomial(
      OrderedVariableList([x,y]),SparseMultivariatePolynomial(Fraction(
      Integer),OrderedVariableList([a,b])))

(13)