[src/jn.c] added notes for correctness of the algorithm used

git-svn-id: svn://scm.gforge.inria.fr/svn/mpfr/trunk@7420 280ebfd0-de03-0410-8827-d642c229c3f4

1 files changed, 3 insertions, 0 deletions

diff --git a/src/jn.c b/src/jn.c
index 8e9201812..af51cf2b8 100644
--- a/src/jn.c
+++ b/src/jn.c
@@ -195,6 +195,8 @@ mpfr_jn (mpfr_ptr res, long n, mpfr_srcptr z, mpfr_rnd_t r)
       if (absn > 0)
         mpfr_div_2ui (t, t, absn, MPFR_RNDN);
       mpfr_set (s, t, MPFR_RNDN);
+      /* note: we assume here that the maximal error is proportional to
+         2^exps, which is true also in the case where s=0 */
       exps = MPFR_IS_ZERO (s) ? MPFR_EMIN_MIN : MPFR_GET_EXP (s);
       expT = exps;
       for (k = 1; ; k++)
@@ -209,6 +211,7 @@ mpfr_jn (mpfr_ptr res, long n, mpfr_srcptr z, mpfr_rnd_t r)
               mpfr_div_ui (t, t, k, MPFR_RNDN);
               mpfr_div_ui (t, t, k + absn, MPFR_RNDN);
             }
+          /* see above note */
           exps = MPFR_IS_ZERO (s) ? MPFR_EMIN_MIN : MPFR_GET_EXP (t);
           if (exps > expT)
             expT = exps;