GAP Manual: 13.5. IsCyc

IsCyc( obj )

returns true if obj is a cyclotomic, and false otherwise. Will signal an error if obj is an unbound variable.

    gap> IsCyc( 0 ); IsCyc( E(3) ); IsCyc( 1/2 * E(3) ); IsCyc( IsCyc );
    true
    true
    true
    false

IsCyc is an internal function.