HSL, originally the Harwell Subroutine Library, is a collection of Fortran 77 and 95 codes that address core problems in numerical analysis.
It is primarily developed by the Numerical Analysis Group at the Rutherford Appleton Laboratory with contributions from other experts in the field.
HSL codes are easily recognizable by the format of their names, consisting of two letters followed by two numbers, dating back to early versions of Fortran's limited subroutine name length.
The letters denote a broad classification of the problem they solve, and the numbers serve to distinguish different codes.
Over the intervening years, HSL has striven to maintain a high standard of reliability and has garnered a worldwide reputation as a prime source of numerical software.