Spectra stands for Sparse Eigenvalue Computation Toolkit as a Redesigned
ARPACK. It is a C++ library for large scale eigenvalue problems, built on
top of Eigen, an open source linear algebra library.

Spectra is implemented as a header-only C++ library, whose only dependency,
Eigen, is also header-only. Hence Spectra can be easily embedded in C++
projects that require calculating eigenvalues of large matrices.