A matrix decomposition is a factorization of a matrix into a product of several matrices. While important theoretically when deriving results, they are also important from the computational point of view.
Pastel implements the LU, QR, and Cholesky decompositions.
Matrix Computations, 3rd. ed., Gene H. Golub, Charles F. Van Loan, 1996.
Numerical Recipes: The Art of Scientific Computing, 3rd. ed., William H. Press, Saul A. Teukolsky, William T. Vetterling, Brian P. Flannery, 2007.