ARPACK (ARnoldi PACKage) is a software library for solving large-scale eigenvalue problems, particularly those arising from sparse matrices. It provides efficient algorithms for computing a few eigenvalues and corresponding eigenvectors of a large, sparse, and possibly non-Hermitian matrix. ARPACK offers a range of methods, including the Arnoldi iteration, Lanczos iteration, and implicitly restarted Arnoldi method, to approximate eigenvalues and eigenvectors.