Is there a way to do what the command divisors[] do in a reverse manner instead of incrementing i++ the numbers that will be checked for the division with remainder 0 starting from a given number lets say 848576890394122387 to 0, the only problem I find to use divisors[] as a way to check for primality is that for large number with 10000000 digits it takes too long to reach the maximum divisor, since what I am trying to find are pseudoprimes with factors too high I should do the reverse, decrementing the numbers from a given number and checking for Mod[a,n]=0, I think it would be faster than primeQ that also works its way up....