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3. Lect. Notes in Math.;J. Stakgold,1973
4. H. B. Keller, ?Nonlinearbifurcation,? J. Diff. Eq.,7, No. 3, 417?434 (1970).
5. M. A. Krasnosel'skii et al., Approximate Solution of Operator Equations [in Russian], Nauka, Moscow (1969).