FCPermuteMomentaRules[{p1, p2, ...}] returns a set of rules that contain all possible permutations of the momenta p1, p2, … . This can be useful when working with amplitudes that exhibit a symmetry in some or all of the final state momenta or when trying to find mappings between loop integrals from different topologies.
FCPermuteMomentaRules[{p1, p2}]
f[p1, p2] /. %\{\{\},\{\text{p1}\to \;\text{p2},\text{p2}\to \;\text{p1}\}\}
\{f(\text{p1},\text{p2}),f(\text{p2},\text{p1})\}
FCPermuteMomentaRules[{p1, p2, p3}]
f[p1, p2, p3] /. %
\{\{\},\{\text{p1}\to \;\text{p2},\text{p2}\to \;\text{p1}\},\{\text{p1}\to \;\text{p3},\text{p3}\to \;\text{p1}\},\{\text{p2}\to \;\text{p3},\text{p3}\to \;\text{p2}\},\{\text{p1}\to \;\text{p2},\text{p2}\to \;\text{p3},\text{p3}\to \;\text{p1}\},\{\text{p1}\to \;\text{p3},\text{p2}\to \;\text{p1},\text{p3}\to \;\text{p2}\}\}
\{f(\text{p1},\text{p2},\text{p3}),f(\text{p2},\text{p1},\text{p3}),f(\text{p3},\text{p2},\text{p1}),f(\text{p1},\text{p3},\text{p2}),f(\text{p2},\text{p3},\text{p1}),f(\text{p3},\text{p1},\text{p2})\}