[src/agm.c] Corrected a comment.

[tests/tagm.c] Added tests concerning special values,
  in particular (zero,negative) and symmetric tests.
[doc/mpfr.texi] Corrected mpfr_agm description.

git-svn-id: svn://scm.gforge.inria.fr/svn/mpfr/trunk@8666 280ebfd0-de03-0410-8827-d642c229c3f4

1 files changed, 4 insertions, 1 deletions

diff --git a/src/agm.c b/src/agm.c
index 355fb060b..738ed17df 100644
--- a/src/agm.c
+++ b/src/agm.c
@@ -72,7 +72,10 @@ mpfr_agm (mpfr_ptr r, mpfr_srcptr op2, mpfr_srcptr op1, mpfr_rnd_t rnd_mode)
             }
         }
       else /* a and b are neither NaN nor Inf, and one is zero */
-        {  /* If a or b is 0, the result is +0 since a sqrt is positive */
+        {  /* If a or b is 0, the result is +0, in particular because the
+              result is always >= 0 with our definition (Maple sometimes
+              chooses a different sign for GaussAGM, but it uses another
+              definition, with possible negative results). */
           MPFR_ASSERTD (MPFR_IS_ZERO (op1) || MPFR_IS_ZERO (op2));
           MPFR_SET_POS (r);
           MPFR_SET_ZERO (r);