FCGetFreeIndices[exp, {head1, head2, ...}] returns the list of free (uncontracted) indices from heads head1, head2, …
As always in FeynCalc, Einstein summation convention is implicitly assumed. The function is optimized for large expressions, i.e. it is not so good as a criterion in e.g. Select.
If it is understood that each term in the expression contains the same number of free indices, setting the option First to True can considerably speed up the evaluation.
Overview, FCRenameDummyIndices, Contract, FCGetDummyIndices, DummyIndexFreeQ, FreeIndexFreeQ.
FCI[FV[p, \[Mu]] FV[q, \[Nu]]]
FCGetFreeIndices[%, {LorentzIndex}]\overline{p}^{\mu } \overline{q}^{\nu }
\{\mu ,\nu \}
FCI[FV[p, \[Mu]] FV[q, \[Mu]]]
FCGetFreeIndices[%, {LorentzIndex}]\overline{p}^{\mu } \overline{q}^{\mu }
\{\}
FCI[SUNT[a, b]]
FCGetFreeIndices[%, {SUNIndex}]T^a.T^b
\{a,b\}
FCI[SUNT[a, a]]
FCGetFreeIndices[%, {SUNIndex}]T^a.T^a
\{\}