Levitin, Michael; Shargorodsky, Eugene (2004). "Spectral pollution and second order relative spectra for self-adjoint operators". IMA Journal of Numerical Analysis. 24 (3): 393–416. arXiv:math/0212087. doi:10.1093/imanum/24.3.393.
Levitin, Michael; Shargorodsky, Eugene (2004). "Spectral pollution and second order relative spectra for self-adjoint operators". IMA Journal of Numerical Analysis. 24 (3): 393–416. arXiv:math/0212087. doi:10.1093/imanum/24.3.393.
Pokrzywa, Andrzej (1979). "Method of orthogonal projections and approximation of the spectrum of a bounded operator". Studia Mathematica. 65: 21–29. doi:10.4064/sm-65-1-21-29.