... other hand, thanks to the monotonicity of the principal eigenvalue with respect to the potential (cf. Proposition 3.3 of S. Cano-Casanova and J. L6pez-G6mez5) and with respect to the weight ... am delighted to thank my colleague and friend J. Lbpez-Gbmez, for invit- ing to me to contribute with this paper in this memory in tribute to the great mathematician, colleague and friend, ... the monotonicity of the principal eigenvalue with respect to the potential and with respect to the weight on the boundary (cf. S. Cano-Casanova and J. L6pez-G6mez5), that CJ?[DuF(X, 4,...