The GNU Scientific Library (GSL) is a free numerical library for C and C++ programmers. It provides over 1,000 routines for solving mathematical problems in science and engineering. Written by the developers of GSL this reference manual is the definitive guide to the library. The GNU Scientific Library is free software, distributed under the GNU General Public License (GPL). All the money raised from the sale of this book supports the development of the GNU Scientific Library. This is the third edition of the manual, and corresponds to version 1.12 of the library (updated January 2009).