Published online by Cambridge University Press: 20 November 2018
The eigenvalue problem  
 $-{{\Delta }_{p}}u-{{\Delta }_{q}}u=\lambda {{\left| u \right|}^{q-2}}u$  with  
 $p\,\in \,\left( 1,\,\infty\right),\,q\,\in \,\left( 2,\,\infty\right),\,p\ne \,q$  subject to the corresponding homogeneous Neumann boundary condition is investigated on a bounded open set with smooth boundary from  
 ${{\mathbb{R}}^{N}}$  with  
 $N\,\ge \,2$ . A careful analysis of this problem leads us to a complete description of the set of eigenvalues as being a precise interval  
 $\left( {{\lambda }_{1,}}+\infty\right)$  plus an isolated point  
 $\lambda \,=\,0$ . This comprehensive result is strongly related to our framework, which is complementary to the well-known case  
 $p\,=\,q\,\ne \,2$  for which a full description of the set of eigenvalues is still unavailable.