The principal eigenvalue problem for a strongly anisotropic second-order elliptic operator

We consider an elliptic operator in which the second-order term is very small in one direction. In this regime, we study the behaviour of the principal eigenfunction and of the principal eigenvalue. Our first result deals with the limit of the principal eigenfunction and is shown with a representation of the principal eigenfunction as a quasi-stationary distribution. Subsequent results deal with the limit of the principal eigenvalue and are shown using Hamilton-Jacobi equations.