[1] K. NAJZAR: 
On the method of least squares of finding eigenvalues of some symmetric operators. Comm. Math. Univ. Carolinae 9 (1968), 311-323. 
MR 0236740 | 
Zbl 0172.17501[2] K. NAJZAR: 
On the method of least squares of finding eigenvalues and eigenfunctions of some symmetric operators. Comm. Math. Univ. Carolinae 11 (1970), 449-462. 
MR 0276807 | 
Zbl 0211.16402[3] K. NAJZAR: 
Error-estimates for the method of least squares of finding eigenvalues and eigenfunctions. Comm. Math. Univ. Carolinae 11 (1970), 463-479. 
MR 0276808 | 
Zbl 0211.16501[4] S. G. MICHLIN: Čislennaja realizacija variacionnych metodov. 1966.
[5] N. I. ACHIEZER I. M. GLASMANN: Theorie der linearen Operatoren in Hilbert-Raum. 1960.