Semilinear problems with a non-symmetric linear part having an infinite dimensional kernel. Berkovits, J.; Fabry, C. — 2004 Portugaliae Mathematica. Nova Série
An extension of the topological degree in Hilbert space. Berkovits, J.; Fabry, C. — 2005 Abstract and Applied Analysis
Periodic solutions of nonlinear vibrating beams. Berkovits, J.; Leinfelder, H.; Mustonen, V. — 2003 Abstract and Applied Analysis