PARI/GP is a widely used Computer Algebra System designed for fast computations in number theory, but also contains a large number of other useful functions to compute with mathematical entities such as matrices, power series, algebraic or p-adic numbers, etc., and a lot of transcendental functions. PARI is also available as a C library to allow for faster computations.
Citation:
@preamble{"\usepackage{url}"}
@manual{PARI,
title = "{PARI/GP, version {\tt 2.1.5}}",
note = "\url{http://pari.math.u-bordeaux.fr/}",
address = {Bordeaux},
year = 2003
}
% no author mentioned: using "The PARI group" was not too informative ]