PARI/GP is a cross platform and open-source computer algebra system designed for fast computations in number theory: factorizations, algebraic number theory, elliptic curves, modular forms, L functions... It also contains a wealth of functions to compute with mathematical entities such as matrices, polynomials, power series, algebraic numbers, etc., and a lot of transcendental functions as well as numerical summation and integration routines. PARI is also available as a C library to allow for faster computations.