InverseDerivations( X, i )
When T[i] is a regular derivation, this function returns the
position j such that T[j] is the inverse of T[i] in the
Whitehead group. When T[i] is not regular, a list of values j is
returned such that the inverse semigroup condition xyx = x, ; yxy =
y is satisfied, where x = T[i], y = T[j]. Notice that derivation 8
has order 3 and derivation 15 as inverse.
gap> inv4 := InverseDerivations( chi4 );
[ 4 ]
gap> inv8 := InverseDerivations( chi8 );
[ 7, 8, 9 ]
GAP 3.4.4