- guaranteed upper and lower bounds for an arbitrary simple eigenvalue, multiple eigenvalue, or a cluster of eigenvalues;
- guaranteed energy error bounds on the associated eigenvector(s);
- efficiency (optimal convergence rate) and polynomial-degree robustness;
- multiplicative constant tending to one under an elliptic regularity assumption on the corresponding
- inexact algebraic solvers taken into account;
- unified framework covering basically any numerical method (conforming, nonconforming, discontinuous Galerkin, and mixed finite elements);
- abstract framework for any second-order self-adjoint elliptic linear operator with compact resolvent.