Fix convergence of Li_projection at ±I.
authorStefan Weinzierl <weinzierl@uni-mainz.de>
Thu, 11 Aug 2016 21:55:59 +0000 (23:55 +0200)
committerRichard Kreckel <kreckel@ginac.de>
Thu, 11 Aug 2016 21:55:59 +0000 (23:55 +0200)
For arguments near ±I, direct summation does not practically converge.
Use the Bernoulli transformation instead.

Thanks to Peter Banks for reporting this:
<http://www.cebix.net/pipermail/ginac-list/2016-August/002085.html>

ginac/inifcns_nstdsums.cpp

index e21e311..67b06ba 100644 (file)
@@ -337,10 +337,10 @@ cln::cl_N Li_projection(int n, const cln::cl_N& x, const cln::float_format_t& pr
                        // the switching point was empirically determined. the optimal point
                        // depends on hardware, Digits, ... so an approx value is okay.
                        // it solves also the problem with precision due to the u=-log(1-x) transformation
-                       if (cln::abs(cln::realpart(x)) < 0.25) {
-                               
+                       if (cln::abs(x) < 0.25) {
                                return Li2_do_sum(x);
                        } else {
+                               // Li2_do_sum practically doesn't converge near x == ±I
                                return Li2_do_sum_Xn(x);
                        }
                } else {
@@ -366,9 +366,10 @@ cln::cl_N Li_projection(int n, const cln::cl_N& x, const cln::float_format_t& pr
                if (cln::realpart(x) < 0.5) {
                        // choose the faster algorithm
                        // with n>=12 the "normal" summation always wins against the method with Xn
-                       if ((cln::abs(cln::realpart(x)) < 0.3) || (n >= 12)) {
+                       if ((cln::abs(x) < 0.3) || (n >= 12)) {
                                return Lin_do_sum(n, x);
                        } else {
+                               // Li2_do_sum practically doesn't converge near x == ±I
                                return Lin_do_sum_Xn(n, x);
                        }
                } else {