Geodesic
Optimal approximations in the eigenvalue problem for the Ritz and Bubnov–Galerkin methods
Žurnal vyčislitelʹnoj matematiki i matematičeskoj fiziki, Tome 23 (1983) no. 5, pp. 1230-1233 
S. N. Kukudzhanov. Optimal approximations in the eigenvalue problem for the Ritz and Bubnov–Galerkin methods. Žurnal vyčislitelʹnoj matematiki i matematičeskoj fiziki, Tome 23 (1983) no. 5, pp. 1230-1233. http://geodesic.mathdoc.fr/item/ZVMMF_1983_23_5_a19/
@article{ZVMMF_1983_23_5_a19,
     author = {S. N. Kukudzhanov},
     title = {Optimal approximations in the eigenvalue problem for the {Ritz} and {Bubnov{\textendash}Galerkin} methods},
     journal = {\v{Z}urnal vy\v{c}islitelʹnoj matematiki i matemati\v{c}eskoj fiziki},
     pages = {1230--1233},
     year = {1983},
     volume = {23},
     number = {5},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/ZVMMF_1983_23_5_a19/}
}
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Voir la notice de l'article provenant de la source Math-Net.Ru

A method is described for finding the best (in a certain sense) approximations of the eigenvalues for linear operator equations of the type $Au=\lambda Bu$, when they are solved by the Ritz and the Bubnov–Galerkin methods. The problem of optimal approximations is stated thus: given the system of coordinate functions $\{\varphi_n\}$, it is required to find, among all the coordinate elements, the $k$ elements for which the divergence $\delta^{(k)}$ between the exact absolute value of the eigenvalue $|\lambda|$ and its $k$-th approximation $|\lambda^{(k)}|$ is minimal, i. e. $|\lambda^{(k)}|-|\lambda|=\min\delta^{(k)}$.