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## [Bug-gsl] GSL Interpolation

**From**: |
Andreas Ipp |

**Subject**: |
[Bug-gsl] GSL Interpolation |

**Date**: |
Thu, 05 Jul 2007 11:15:05 +0200 |

**User-agent**: |
Thunderbird 2.0.0.0 (X11/20070326) |

Hello,

`I have a recommendation for improving the spline interpolation
``documentation:
`

`One should mention that x-values have to be given in increasing order...
``i.e. x[0] < x[1] < x[2] < ...
`

`I just had the problem where I wanted to include a spline for a function
``that I get from my calculation as e.g.
`x[0] = 10; y[0] = ...
x[1] = 9;
x[2] = 8;
etc. so that x[0] > x[1] > x[2]...

`Since I was new to interpolation, at first I could not understand the
``error message:
`
gsl: interp.c:134: ERROR: interpolation error
Default GSL error handler invoked.
Aborted

`Also, since we only had the library and not the source code installed at
``my work place, I could not check which line caused the error.
``Finally, by trial and error, I realized that reverse order of x is not
``allowed for interpolation, but it would have been nice, if it had been
``mentioned somewhere in the manual (in other systems, it doesn't matter
``whether x is sorted upward or downward...)
`
Sincerely,
Andreas Ipp

**[Bug-gsl] GSL Interpolation**,
*Andreas Ipp* **<=**