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Re: symbolic matrix manipulation

ashwin.tulapurkar at wrote:
> Hi,
> I am trying to simplify the following matrix expression:
> a.b.b.a with the rule: replace a.b by (b.a+1). So I expect the final
> output to be
> a.b.b.a --> (b.a+1).b.a --> b.a.b.a+b.a --> b.(b.a+1).a+b.a -->
> b.b.a.a + 2 b.a
> Can you tell me how to do this?
> Thanks.
> -Ashwin

This really looks more like something from a general setting of 
noncommutative algebra. I'll recast that way so as to avoid explicit use 
of matrix operators such as Dot, which will not handle this well. I'm 
adapting code from

I use a "multiplication" operator I call ncTimes. I designate the symbol 
s for variables to be treated as noncommuting. We impose a commutator 
relation that s[i] s[j] = s[j] s[i] + 1 for j>i (so we just need to make 
sure b correspnds to an s ariable with larger index than a).

ncTimes[] := 1
ncTimes[a_] := a
ncTimes[a___,x_+y_,b___] := ncTimes[a,x,b] + ncTimes[a,y,b]
ncTimes[a___,ncTimes[b_,c__],d___] := ncTimes[a,b,c,d]
ncTimes[a___,i_Integer*c_,b___] := i*ncTimes[a,c,b]
ncTimes[a___,i_Integer,b___] := i*ncTimes[a,b]
ncTimes[a___,s[i_Integer],s[j_Integer],b___] /; j>i :=

a = s[1];
b = s[2];

Your example, in this setting, is done as below.

In[10]:= ncTimes[a,b,b,a]
Out[10]= 2 ncTimes[s[2], s[1]] + ncTimes[s[2], s[2], s[1], s[1]]

Daniel Lichtblau
Wolfram Research

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