From the Introduction:
The GNU Scientific Library (GSL) is a numerical library for C and C++ programmers. It is free software under the GNU General Public License.
The library provides a wide range of mathematical routines such as random number generators, special functions and least-squares fitting. There are over 1000 functions in total with an extensive test suite.
This can save your time, effort, and complexity of your C/C++ code (instead of coding on your own, you can call functions in the scientific library, which makes your code compact and decipherable).
[Added] See my pyIMSL where I made links to IMSL manuals. I have no guts to print Fortran and C/C++ versions for the sake of our earth.
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