id	summary	reporter	owner	description	type	status	milestone	component	version	severity	resolution	keywords	cc
6307	problem in bessel_jn	phzwart@…	John Maddock	"In bessel_jn (math/special_functions/detail) line 75 reads:

if((tools::max_value<T>() - fabs(prev)) / fabs(fact) < fabs(current))

fact is defined in line 74:

T fact = 2 * k / x;

when k is 1 and x > 2 , fact < 1 resulting in 

((tools::max_value<T>() - fabs(prev)) / fabs(fact) 

to be bigger then tools::max_value<T>(), resulkting in an error.

I have change line 75 into 

if((tools::max_value<T>() - fabs(prev)) < fabs(fact) * fabs(current))

and made the same change for line 100.

With this fix, the code runs and produces (in my limited testing) the same numbers as octave does. I am not sure if it is a general fix though.


The problem occurred on this architecture:

Linux syrah.als.lbl.gov 2.6.35.14-97.fc14.x86_64 #1 SMP Sat Sep 17 00:15:37 UTC 2011 x86_64 x86_64 x86_64 GNU/Linux


svn diff gives this:

===================================================================
--- bessel_jn.hpp       (revision 76082)
+++ bessel_jn.hpp       (working copy)
@@ -72,7 +72,7 @@
         for (int k = 1; k < n; k++)
         {
             T fact = 2 * k / x;
-            if((tools::max_value<T>() - fabs(prev)) / fabs(fact) < fabs(current))
+            if((tools::max_value<T>() - fabs(prev)) < fabs(fact) * fabs(current))
             {
                scale /= current;
                prev /= current;
@@ -97,7 +97,7 @@
         for (int k = n; k > 0; k--)
         {
             T fact = 2 * k / x;
-            if((tools::max_value<T>() - fabs(prev)) / fact < fabs(current))
+            if((tools::max_value<T>() - fabs(prev)) < fabs(fact) * fabs(current))
             {
                prev /= current;
                scale /= current;











"	Bugs	closed	To Be Determined	math	Boost Development Trunk	Showstopper	fixed