Principal eigenvalues for problems with indefinite weight function on RN

We investigate the existence of positive principal eigenvalues of the problem −Δu(x)=λg(x)u for x∈R n ; u(x)→0 as x→∞ where the weight function g changes sign on R n . It is proved that such eigenvalues exist if g is negative and bounded away from 0 to ∞ or if n≥3 and |g(x)| is sufficiently small at ∞ but do not exist if n=1 or 2 and ∫ Rn g (x)dx>0