Free Download Subgroups of Teichmuller modular groups… By Nikolai V. Ivanov Publisher American Mathematical Society 1992 ISBN 0821845942,9780821845943..

Free Download Linear and Nonlinear Perturbations of the Operator… By V. G. Osmolovskii Publisher American Mathematical Society 1997 ISBN 0-8218-0586-X,9780821805862..