BIBCYT http://bibcyt.ucla.edu.ve/cgi-win/be_alex.exe?Ultimo&Nombrebd=BIBCYT UCLA - Biblioteca de Ciencias y Tecnologia Felix Morales Bueno Stevens, A. P Principal Papanicolaou, G. P Principal Heinze, S. P Principal Variational Principles for Propagation Speeds in Inhomogeneous Media pp. 129-148