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2. A bifurcation theory for nonlinear elliptic partial differential equations and related systems;Berger,1969
3. Multiple solutions of nonlinear operator equations arising from the calculus of variations;Berger,1970
4. The category of a map and of a cohomology class;Berstein;Fund. Math.,1961
5. Existence theorems for nonlinear partial differential equations;Browder,1970