Bifurcation into spectral gaps for strongly indefinite Choquard equations

: We consider the semilinear elliptic equations where I α is a Riesz potential, p ∈ ( N + αN , N + α N − 2 ), N ≥ 3, and V is continuous periodic. We assume that 0 lies in the spectral gap ( a, b ) of − ∆ + V . We prove the existence of infinitely many geometrically distinct solutions in H 1 ( R N ) for each λ ∈ ( a, b ), which bifurcate from b if N + αN < p < 1 + 2+ αN . Moreover, b is the unique gap-bifurcation point (from zero) in [ a, b ]. When λ = a , we find infinitely many geometrically distinct solutions in H 2 loc ( R N ). Final remarks are given about the eventual occurrence of a bifurcation from infinity in λ = a . 35Q55, 47J35.