ARPACK, the ARnoldi PACKage, is a numericalsoftware library written in FORTRAN 77 for solving large scale eigenvalue problemsin the matrix-free fashion. The package is designed to compute a few eigenvalues and correspondingeigenvectors of large sparse or structured matrices, using theImplicitly Restarted Arnoldi Method (IRAM)or, in the case of symmetric matrices, the corresponding variant of theLanczos algorithm.It is used by many popular numerical computing environments such as SciPy, Mathematica, GNU Octaveand MATLAB to provide this functionality.