GMP gives wrong (negative) results

[Torbjörn Granlund]

"V. Nicolai Friedhoff" <vnf at gmx.de> writes:

  > Let me rephrase that: your program is correct for some parameters,
  > therefore you know it is correct for all parameters.
  >
  > Skeptical programmers might not find that reasoning convincing.
  
  Nope, I said that I am sure, not that I know. But I am very sure because I
  [snip]
  
I took a look at your program and I immediately spotted several bugs (a
syntax error, a handful of memory leaks, and a curious lack of concerns
of precision).  So whether you're sure, very sure, or merely know that
your program is correct, to me your program seems pretty buggy...

  2.-4. Could I find out if any of this is the case?

I'd say to fix your code first, then look elsewhere.

  6. I only include gmp.h and gmpfrxx.h, nothing else.

So you're actually not using GMP's mpf at all, but MPFR's mpf layer?

  7. Its a pretty new workstation, so I doubt that :P

OK.  Did you consider item 5?  :-)
  
  Do you mean regular doubles, without GMP? I tried that and the program
  fails much earlier in that case, like at x=5, giving me again large
  negative results. The problem I think is that some positive terms in the
  sum are too small or large, so they get neglected, while some other
  negative terms are valid. This adds up to a negative end result, far from
  what the sum should actually be. I would interpret this collective behavior
  as a suggestion that my implementation and maths are correct, but somehow
  all the platforms get the same issue at largeish numbers (x=5 for c++
  doubles, x=30 for GMP, x=120 for Mathematica) with that series.
  
Isn't a more reasonable conclusion then that you have problems with what
numerical analysts call cancellation, than that "GMP gives wrong
results"?

-- 
Torbjörn
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