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I work with traces of long matrix products and I want mathematica to employ the cyclicity when simplifying.

I use this redefinition of the non-commutative multiplication:

Unprotect[NonCommutativeMultiply];
Clear[NonCommutativeMultiply]

(*Factor out numerics-- could generalize to some ScalarQ*)
nc : NonCommutativeMultiply[a__] /; MemberQ[{a}, _?NumericQ] := 
 NCMFactorNumericQ[NCM[a]] /. NCM -> NonCommutativeMultiply

(*Simplify Powers*)    
b___ ** a_^n_. ** a_^m_. ** c___ := 
 NCM[b, a^(n + m), c] /. NCM -> NonCommutativeMultiply

(*Minus*)
NonCommutativeMultiply[a___, b_Times, c___] := 
 NCMFactorNumericQ[NCM[a, Sequence @@ b, c]] /. 
  NCM -> NonCommutativeMultiply
Protect[NonCommutativeMultiply];

Unprotect[NCM];
Clear[NCM]
NCMFactorNumericQ[nc_NCM] := 
 With[{pos = Position[nc, _?NumericQ, 1]}, 
  Times @@ Extract[nc, pos] Delete[nc, pos]]
NCM[a_] := a
NCM[] := 1
Protect[NCM];

which was given as an answer to this question and is very helpful, but I want it to be able to cancel A**B**C with -B**C**A (but only if these matrices stand alone, i.e. I don't want it to cancel A**B**C**D with -B**C**A**D). Any help would be greatly appreciated.

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  • $\begingroup$ Beware! By default, ** is associative, but one implication of the last statement is that your multiplication is not. Thus, even an expression like A**B**C is not well-defined, because it could mean either (A**B)**C or A**(B**C). You probably should not be thinking of your manipulations with traces as resulting from a binary operation at all. $\endgroup$
    – whuber
    Dec 13, 2012 at 15:25
  • 1
    $\begingroup$ Good point, @whuber . I was thinking something along the lines of letting it check if what is left and right is a one (then it can cycle) or something else (then it can't). On the other hand, I don't really need it to be encoded in the multiplication itself. I'd be perfectly happy with a newly defined trace that I can apply on my expressions and that knows that cyclic permutations are identical. $\endgroup$
    – Sebastik
    Dec 13, 2012 at 16:41

1 Answer 1

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NCAlgebra supports a trace type operator. It will automatically reduce

tr[a ** b ** c - b ** c ** a]

to

0

and

tr[a ** b ** c ** d - b ** c ** a ** d]

tr[a ** b ** c ** d] - tr[a ** d ** b ** c]

It achieves that by automatically canonizing the sorting of the arguments or tr.

P.S.: We are in the process of updating NCAlgebra so you might want to try our latest beta version from here, which you can install as a paclet.

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