[Boost-bugs] [Boost C++ Libraries] #6307: problem in bessel_jn

Subject: [Boost-bugs] [Boost C++ Libraries] #6307: problem in bessel_jn
From: Boost C++ Libraries (noreply_at_[hidden])
Date: 2011-12-20 19:39:56

#6307: problem in bessel_jn
-------------------------------------+--------------------------------------
 Reporter:  phzwart@…                |       Owner:  johnmaddock
     Type:  Bugs                     |      Status:  new        
Milestone:  To Be Determined         |   Component:  math       
  Version:  Boost Development Trunk  |    Severity:  Showstopper
 Keywords:                           |  
-------------------------------------+--------------------------------------
 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;

-- 
Ticket URL: <https://svn.boost.org/trac/boost/ticket/6307>
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