GAP Manual: 13.7. NofCyc

NofCyc( z )
NofCyc( list )

returns the smallest positive integer n for which the cyclotomic z is resp. for which all cyclotomics in the list list are contained in Q_n = Q( e^(frac(2 pii)(n)) ) = Q( E(n) ).

    gap> NofCyc( 0 ); NofCyc( E(10) ); NofCyc( E(12) );
    1
    5
    12

NofCyc is an internal function.