**Next message:**Vladyslav Shtabovenko: "Re: The Tr is terriblely slow in FC6.0"**Previous message:**Sam: "Re: trace of four gamma matrices is wrong sometimes"**In reply to:**Sam: "Re: trace of four gamma matrices is wrong sometimes"**Next in thread:**Sam: "Re: trace of four gamma matrices is wrong sometimes"**Messages sorted by:**[ date ] [ thread ] [ subject ] [ author ]**Mail actions:**[ respond to this message ] [ mail a new topic ]

Hi Sam,

the thing is that in FeynCalc all non-commutative objects (like Dirac or

SU(N) matrices) must be separated by a "." simply because in Mathematica

the usual multiplication "a*b===b*a" is commutative while

the "." multiplication is not "a.b=!=b.a".

In your code you enter Dirac matrices without the "." as if the ordering

doesn't matter, so no wonder that the result is ambiguous. With

Tr[DiracMatrix[a]. DiracMatrix[b]. DiracMatrix[c]. DiracMatrix[d]]

and

Tr[DiracMatrix[a]. DiracMatrix[\[Beta]].

DiracMatrix[c] .DiracMatrix[\[Delta]]]

you get the correct result in both cases.

BTW, you can also use "GA" instead of DiracMatrix

(see <https://github.com/FeynCalc/feyncalc/wiki/FAQ#fci_fce>

for more details) which is much more compact:

Tr[GA[a,b,c,d]]===(Tr[GA[a,\[Beta],c,\[Delta]]]/.{\[Beta]->b,\[Delta]->d})

Cheers,

Vladyslav

On 17/12/14 23:26, Sam wrote:

*> I should mention that I am using Mathematica 10.0.2.0 (released very recently) on a Mac OS X 10.10, with FeynCalc 8.2.0 and FeynArts 3.7 patched for use with FeynCalc.
*

*>
*

*> Thanks again,
*

*> Sam
*

*>
*

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