%% R.E. Showalter: 


\begin{thebibliography}{99}

\bibitem{adams} R.A. Adams, 
{\em Sobolev Spaces}, Academic Press, 1976.

\bibitem{agmon} S. Agmon, 
{\em Lectures on Elliptic Boundary Value Problems}, Van Nostrand, 1965. 

\bibitem{aubin} J.P. Aubin, 
{\em Approximation of Elliptic Boundary Value Problems}, Wiley, 1972. 

\bibitem{brezis} H. Brezis, 
{\em Operateurs Maximaux Monotones}, North-Holland Math. Studies 5, 1973. 

\bibitem{browder} F.E. Browder, 
{\em Nonlinear Operators and Nonlinear Equations of Evolution in Banach 
Spaces}, Proc. Symp. Pure Math., {\bf 18}, part~2, Amer. Math. Soc., 1976. 

\bibitem{butzer} P. Butzer and H. Berens, 
{\em Semi-groups of Operators and Approximations}, Springer, 1967. 

\bibitem{carasso} A. Carasso and A. Stone (editors), 
{\em Improperly Posed Boundary Value Problems}, Pitman, 1975. 

\bibitem{carroll} R.W. Carroll, 
{\em Abstract Methods in Partial Differential Equations}, Harper-Row, 1969. 

\bibitem{carrshow} R.W. Carroll and R.E. Showalter, 
{\em Singular and Degenerate Cauchy Problems}, Academic Press, 1976. 

\bibitem{cea} J. Cea, 
{\em Optimization. Theorie et Algorithmes}, Dunod, 1971. 

\bibitem{ciarlet} P.G. Ciarlet, 
{\em Numerical Analysis of the Finite Element Method for Elliptic 
Boundary Value Problems}, North-Holland, 1977. 

\bibitem{colton} D.L. Colton, 
{\em Partial Differential Equations in the Complex Domain}, Pitman, 1976. 

\bibitem{courant} R. Courant and D. Hilbert, 
{\em Methods of Mathematical Physics}, Vol.2, Wiley, 1962. 

\bibitem{daniel} J. Daniel, 
{\em Approximate Minimization of Functionals}, Prentice Hall, 1970. 

\bibitem{dickey} R.W. Dickey, 
{\em Bifurcation Problems in Nonlinear Elasticity}, Pitman, 1976. 

\bibitem{duvaut} G. Duvaut and J.L. Lions, 
{\em Les Inequations en Mecanique et en Physique}, Dunod, 1972.  

\bibitem{ekeland} I. Ekeland and R. Temam, 
{\em Analyse Convexe et Problemes Variationnels}, Dunod, 1974. 

\bibitem{fichera} G. Fichera (editor), 
{\em Trends in Applications of Pure Mathematics to Mechanics}, Pitman, 1976. 

\bibitem{friedman} A. Friedman, 
{\em Partial Differential Equations}, Holt-Rinehart-Winston, 1969. 

\bibitem{glow} R. Glowinski, J.L. Lions and R. Tremolieres, 
{\em Analyse Numerique des Inequations Variationnelles}, Dunod, 1976. 

\bibitem{goldstein} J.R. Goldstein, 
{\em Semi-groups of Operators and Abstract Cauchy Problems}, 
Tulane University, 1970. 

\bibitem{hellwig} G. Hellwig, 
{\em Differential Operators of Mathematical Physics}, Addison-Wesley, 1967. 

\bibitem{hille} E. Hille and R.S. Phillips, 
{\em Functional Analysis and Semigroups}, 
Amer. Math. Soc. Coll. Publ., Vol.31, 1957. 

\bibitem{hormander} L. Hormander, 
{\em Linear Partial Differential Operators}, Springer, 1963. 

\bibitem{horvath} J. Horvath, 
{\em Topological Vector Spaces and Distributions}, Vol.1, Addison-Wesley, 1967. 

\bibitem{jeffrey} A. Jeffrey, 
{\em Quasilinear Hyperbolic Systems and Waves}, Pitman, 1976. 

\bibitem{ladas} G. Ladas and V. Lakshmikantham, 
{\em Differential Equations in Abstract Spaces}, Academic Press, 1972. 

\bibitem{ladySU} O. Ladyzenskaya, V. Solonnikov and N. Uralceva, 
{\em Linear and Quasilinear Equations of Parabolic Type}, 
Izd. Nauka, 1967. 

\bibitem{lax} P. Lax and R.S. Phillips, 
{\em Scattering Theory}, Academic Press, 1967. 

\bibitem{lionsDO} J.L. Lions,  
{\em Equations Differentielles-Operationnelles}, Springer, 1961. 

\bibitem{lionsPDE} J.L. Lions, 
{\em Optimal Control of Systems Governed by Partial Differential Equations}, 
Springer, 1971. 

\bibitem{lionsQM} J.L. Lions, 
{\em Quelques Methods de Resolution des Problemes aux Limites Non-lineares}, 
Dunod, 1969. 

\bibitem{lionsMag} J.L. Lions and E. Magenes, 
{\em Non-homogeneous Boundary Value Problems and Applications}, Vol.1, 
Springer, 1972. 

\bibitem{martin} R.H. Martin, 
{\em Nonlinear Operators and Differential Equations in Banach Spaces}, 
Wiley, 1976. 

\bibitem{mizohata} S. Mizohata, 
{\em The Theory of Partial Differential Equations}, Cambridge, 1973. 

\bibitem{necas} J. Necas, 
{\em Les Methodes Directes dans la Theorie des Equations aux Derivees 
Partielles}, Masson, 1967. 

\bibitem{odenreddy} J.T. Oden and J.N. Reddy, 
{\em Mathematical Theory of Finite Elements}, Wiley, 1976. 

\bibitem{payne} L. Payne, 
{\em Improperly Posed Problems in Partial Differential Equations}, 
CBMS Series, Soc. Ind. Appl. Math., 1976. 

\bibitem{rocka} R.T. Rockafellar, 
{\em Convex Analysis}, Princeton University Press, 1970. 

\bibitem{schultz} M. Schultz, 
{\em Spline Analysis}, Prentice-Hall, 1973. 

\bibitem{strauss} W.A. Strauss, 
{\em The Energy Method in Nonlinear Partial Differential Equations}, 
Notas de Matematica 47, IMPA, 1969. 

\bibitem{strang} G. Strang and G. Fix, 
{\em An Analysis of the Finite Element Method}, Prentice-Hall, 1973. 

\bibitem{treves} F. Treves, 
{\em Basic Linear Partial Differential Equations}, Academic Press, 1975. 

\bibitem{tychonov} A.N. Tychonov and A.A. Samarski, 
{\em Partial Differential Equations of Mathematical Physics}, 
Holden-Day, 1964. 

\bibitem{vainberg} M.M. Vainberg, 
{\em Variational Method and Method of Monotone Operators in the Theory 
of Nonlinear Equations}, Wiley, 1973. 

\bibitem{widder} D.V. Widder, 
{\em The Heat Equation}, Academic Press, 1975. 

\bibitem{yosida} K. Yosida, 
{\em  Functional Analysis} (4th edition), Springer, 1974. 

\end{thebibliography}