Existence of solutions for an elliptic equation with indefinite weight

We consider the elliptic equation with indefinite weight −Δu=V(x)u+f(x,u),u∈H1(RN), where V(x) is a function possibly changing sign in RN; under certain assumptions on f(x,u), we obtain the existence of m−n pairs of nontrivial solutions, where m>n, and the m and n are all integers.