[alexandria-devel] Subfactorial is wrong starting from 11

[Sergey Dymchenko]

I've found that it's floating point rounding error.
Changing (exp 1) to (exp 1d0) fixes the problem for numbers below 19.

But I think 18 is still too small for a library.
It could be just predefined constants table with more success.
Maybe subfactorial should be rewritten to give exact answers? (at
least (exp 1) should be changed to (exp 1d0) ASAP, I believe, because
otherwise subfactorial gives wrong answers even for 11).

On Sun, Jan 23, 2011 at 12:07 AM, Sergey Dymchenko <kit1980 at gmail.com> wrote:
> Hi!
>
> As far as I understand Alexandria uses approximate formula for
> subfactorial, and it became wrong very fast.
> Subfactorial of 10 is correct, but for 11 it's wrong.
>
>           alexandria                exact
> 11        14684571         14684570
> 12      176214848        176214841
> 13    2290792960      2290792932
>