Re: [q-lang-users] Problem using function "isprime"

[<Dr....@t-...>]

we...@gm... wrote:
> ==> isprime 1000003;
> isprime 1000003
> 
> It shouldn't be a problem to determine that 1000003 is a prime?

Well, this uses GMP's probabilistic prime test (Miller-Rabin algorithm), 
which may fail (in which case the Q function also fails). As a remedy, 
you can try to increase the clib::ISPRIME_REP value, or add your own 
test which catches the few cases when the Miller-Rabin test fails. See 
the chapter on "Clib", Section "Additional Integer Functions" in the 
manual for details. Here's a suitable definition which you can add to 
your program, copied straight from the docs:

      isprime N:Int = true if N = 2;
                    = false if N and 1 = 0; // even number
                    = try_div 3 (intsqrt N) N otherwise;

      try_div L M N = true if L > M;
                    = false if N mod L = 0;
                    = try_div (L+2) M N otherwise;

-- 
Dr. Albert Gr"af
Dept. of Music-Informatics, University of Mainz, Germany
Email:  Dr....@t-..., ag...@mu...
WWW:    http://www.musikwissenschaft.uni-mainz.de/~ag