[gnumeric] gnm_digamma: fix step-down range to use either series 2 or 3 at the end.

[Morten Welinder <mortenw src gnome org>]

commit 5a64e1b4ddd24ab248d4ea302497992892189f27
Author: Morten Welinder <terra gnome org>
Date:   Sat Dec 22 21:44:10 2018 -0500

    gnm_digamma: fix step-down range to use either series 2 or 3 at the end.
    
    We end up somewhere in the range [x0,x0+1].  For the first third of that
    range it's better to use series 2.

 src/sf-gamma.c | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)
---
diff --git a/src/sf-gamma.c b/src/sf-gamma.c
index 6a0604c86..a2d260e72 100644
--- a/src/sf-gamma.c
+++ b/src/sf-gamma.c
@@ -1536,7 +1536,7 @@ fixup_upper_real (gnm_complex *res, gnm_complex a, gnm_complex z)
 
        if (GNM_CREALP (z) && z.re < 0 &&
            GNM_CREALP (a) && a.re != gnm_floor (a.re)) {
-               // Everything in the lower power series is real expect z^a
+               // Everything in the lower power series is real except z^a
                // which is not.  So...
                // 1. Flip to lower gamma
                // 2. Assume modulus is correct
@@ -1914,24 +1914,24 @@ gnm_digamma (gnm_float x)
                return gnm_digamma_series_1 (x);
 
        if (x < x0a + 1.0 / 3.0)
-               // Series for range [1.13,1.79] around x0
+               // Series for range [1.13;1.79] around x0
                // Take extra care to compute the difference to x0 with a high-
                // precision version of x0
                return gnm_digamma_series_2 (x, (x - x0a) - x0b);
 
        if (x < x0a + 1)
-               // Series for range [1.79,2.46]
+               // Series for range [1.79;2.46]
                return gnm_digamma_series_3 (x);
 
        if (x < 20) {
-               // Step down to series 3.  All terms are positive so no
+               // Step down to series 2 or 3.  All terms are positive so no
                // cancellation.
                gnm_float sum = 0;
                while (x > x0a + 1) {
                        x--;
                        sum += 1.0 / x;
                }
-               return sum + gnm_digamma_series_3 (x);
+               return sum + gnm_digamma (x);
        }
 
        return gnm_digamma_asymp (x);