documentation issue

Paul Zimmermann Paul.Zimmermann at
Fri Feb 29 10:37:11 CET 2008

>    -- Function: int mpz_invert (mpz_t ROP, mpz_t OP1, mpz_t OP2)
>        Compute the inverse of OP1 modulo OP2 and put the result in ROP.
>        If the inverse exists, the return value is non-zero and ROP will
>        satisfy 0 <= ROP < OP2.  If an inverse doesn't exist the return
>                ^^^^^^^^
>        value is zero and ROP is undefined.
> Is something not working right for |OP2| = 1?  The documentation
> doesn't mention it explicitly, but since its workding is right for
> this case, I don't see any need.  (Just as OP2=4711 does not need
> special wording...)

indeed, the fact that the return value is zero for |OP2|=1 does not agree
with the documentation. Take for example OP1=17 and OP2=1, then ROP=0
satisfies ROP*OP1 = 1 (mod OP2), thus an inverse exists, and according
to the documentation, one should get a non-zero return value, with ROP=0
(this would be the only case where ROP=0).

Conversely, for OP2=0, I don't understand why the return value is non-zero
for OP1=1 or -1, with ROP=1. Most implementations I know of raise a "Division
by zero" error when OP2=0, whatever the value of OP1.

Those two examples show that the documentation should precisely define what
is meant by "inverse of OP1 modulo OP2", especially in those special cases.


More information about the gmp-bugs mailing list