LAPACK is a well-known public domain package written in Fortran77 which provides efficient dense matrix and vector manipulations ranging from simple vector additions to complex matrix decompositions.
LAPACK consists of several hundred hierarchically structured subroutines. At the lowest level of the hierarchy are the Basic Linear Algebra Subroutines or BLAS. Each LAPACK subroutine contains sufficient documentation for its use. In addition there is an on-line HTML manual [1] and a printed manual [2]