How does one calculate 2 ^ -18 ?

Torbjorn Granlund tg at
Wed Jan 7 16:02:37 CET 2009

"Bruce M. Axtens" <bruce.axtens at> writes:

  Please pardon the mathematical naivete: I passed -18 to mpz_pow_ui and 
  the code crashed. No surprises there ("read the friendly manual, you 
  goose!")  So I went back to the manual and tried to get my mind around 
  mpz_powm and related things. I'm still as confused as before. Please, 
  someone, take pity on me and demonstrate in a bit of C how to raise 2 to 
  the negative 18.
2^(-18) is not an integer.  If you calculate it somehow and store it as
an mpz_t, you will get one of the integers 0 or 1, depending on which
functions you use.

A separate problem is the interpretation of -18 as the C type "unsigned
long", which is what mpz_pow_ui expects.  It will be intepreted as a
very large integer (typically 2^32-18 or 2^64-18, depending on your


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