Titensky, M. E. “ A Note on the Stability of the Generalized Ritz Method”.
Rev. Anal. Numér. Théor. Approx.
, vol. 23, no. 2, Aug. 1994, pp. 217-25,
https://ictp.acad.ro/jnaat/journal/article/view/1994-vol23-no2-art10
.