GAP Manual: 51.4. ProjectionMap

ProjectionMap( map )

For each image i under the (necessarily not parametrized) map map, ProjectionMap( map )[i] is the smallest preimage of i.

(We have CompositionMaps( map, ProjectionMap( map ) ) the identity map.)

    gap> ProjectionMap( [1,1,1,2,2,2,3,4,5,5,5,6,6,6,7,7,7] );
    [ 1, 4, 7, 8, 9, 12, 15 ]