  In this paper we consider generalized eigenvalue problems for a family of
operators with a quadratic dependence on a complex parameter. Our model is
$L(\lambda)=-\triangle +(P(x)-\lambda)^2$ in $L^2(\R^d)$ where $P$ is a
positive elliptic polynomial in $\R^d$ of degree $m\geq 2$. It is known that
for $d$ even, or $d=1$, or $d=3$ and $m\geq 6$, there exist $\lambda\in\C$ and
$u\in L^2(\R^d)$, $u\neq 0$, such that $L(\lambda)u=0$. In this paper, we give
a method to prove existence of non trivial solutions for the equation
$L(\lambda)u=0$, valid in every dimension. This is a partial answer to a
conjecture in \cite{herowa}.
