In this paper, we investigate the existence of multiple positive periodic solutions to a class of functional difference equations. We answer the open problems proposed by Y. Raffoul in [Electron. J. Differential Equations 55 (2002) 1–8] and the conditions obtained improve some recent results established there.
It Nike Free 3 will be shown that the number of equivalence classes of embeddings of a 3-connected nonplanar graph into a projective plane coincides with the number of isomorphism classes of planar double coverings of the graph and a combinatorial method to determine the number will be developed.
The effect of vasopressin on the permeability of isolated frog skin to some cations was studied by determining the permeability coefficients of K+, Cs+ and Rb+ in relation to that of Na+, with and without supramaximal doses of vasopressin. The relative permeability coefficients for Cs+ and Rb+ are much lower in the presence that in the absence of vasopressin, while for Nike Lunar K+ it remains unaltered.
The rapid growth of tourism to the USA will continue — at least until the year 2000 and unless upset by economic or other tourism-curtailing catastrophes. This conclusion, based on time-series forecasts and strengthened by a Delphi-type review, makes adequate planning vital if tourism is indeed to realise its full potential.