- guaranteed upper and lower bounds for an arbitrary simple eigenvalue;
- guaranteed energy error bounds on the associated eigenvector;
- efficiency (optimal convergenxce 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).