## #6307 closed Bugs (fixed)

# problem in bessel_jn

Reported by: | Owned by: | John Maddock | |
---|---|---|---|

Milestone: | To Be Determined | Component: | math |

Version: | Boost Development Trunk | Severity: | Showstopper |

Keywords: | Cc: |

### Description

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;

### Attachments (0)

### Change History (2)

### comment:1 Changed 6 years ago by

Resolution: | → fixed |
---|---|

Status: | new → closed |

### comment:2 Changed 6 years ago by

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(In [76094]) Delete unneeded variable. Fixes #6307.