mpfr sin(pi) != 0

Torbjorn Granlund tg at
Thu Mar 1 20:58:08 CET 2012

Jacopo Nespolo <j.nespolo at> writes:

  this is slightly, but not so much, off topic, as it is a problem
  related to mpfr rather than gmp strictly speaking.
  I found myself calculating
             sin(pi * n * x)
  with n in N and x in R. I used mpfr_const_pi for pi.
  Sometimes it happens that n*x = 1, and this screwed the whole
  calculation because i would get 10^-(a lot, according to working
  precision) instead of zero. This was also a particularly unlucky case,
  because it happened to be in the subdiagonal of a sparse matrix,
  surrounded by zeros, thus the diagonalisation algorithms in Eigen
  considered important to keep it.
  Do you think it could be worth it to file a bug, at least to warn
  about the issue in the documentation of the trigonometric functions,
  or it is just expected behaviour and I got fool because I'm a newbie?
After having read and reread your text, I fail to understand what you're
trying to say.

But it seems very clear that whatever problems you might have
encountered, they are not GMP-related.


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