LAPUR, a computer program in FORTRAN-IV, is a mathematical description of the core of a boiling water reactor. Its two linked modules, LAPURX and LAPURW, respectively, solve the steady state governing equations for the coolant and fuel and the dynamic equations for the coolant, fuel, and neutron field in the frequency domain. General implementation descriptions are followed by a detailed description of input and output parameters of LAPURX and LAPURW. Sample inputs are included and stability benchmarks are noted. 12 refs., 2 figs., 2 tabs.