The authors would like to thank very much Jean-Michel Grandmont for valuable comments and suggestions, and for bis constant encouragement. Special grateful thanks to Robin G. de Vilder and Duncan J. Sands for enlightening remarks on our work and for helping with the computer work.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.