GAP Manual: 10.19. PrevPrimeInt

PrevPrimeInt( n )

PrevPrimeInt returns the largest prime which is strictly smaller than the integer n.

Note that PrevPrimeInt uses a probable-primality test (see IsPrimeInt).

    gap> PrevPrimeInt( 541 );
    523
    gap> PrevPrimeInt( 1 );
    -2