Nonlinear Anaiyss, Theory. Methods & Apphcorronr. Vol. 9, No. 11. pp. 1227-1241, 1985 0362-546)(/85 13.W + .W Printed in Great Britain. 0 1985 Pergamon Press Ltd EXISTENCE THEOREMS FOR NONSELFADJOINT SEMILINEAR ELLIPTIC BOUNDARY VALUE PROBLEMS LAMBERTOCESARI Department of Mathematics, University of Michigan, Ann Arbor, Michigan 48109, U.S.A and PATRIZIA PUCCI* Dipartimento di Matematica, Universita degli Studi, 06100 Perugia, Italy (Received 1 April 1984: receiued for publication 25 February 1985) Key words and phrases: Existence of solutions, nonlinear elliptic partial differential equations, non- selfadjoint boundary value problems, the alternative method, square norm topology, finite dimensional kernels and cokemels. 1. INTRODUCTION IN THE present paper we present new existence theorems for weak and strong solutions of problems of the form Ex = Nx, where E is a real elliptic linear differential operator in a bounded domain G of R” with a given system of linear homogeneous conditions, say, BX = 0 on the boundary aG of G and where N is a Nemitsky type nonnecessarily linear operator. We shall make use here of the alternative method, and particularly we shall make use for the elliptic case of new remarks. These remarks suggest that both the auxiliary and bifurcation equations can be analyzed under different topologies, and by a more specific construction of the operator S: Y0 -+ X0. Actually some of these remarks have been already used implicitly in previous papers on the semilinear wave equation in R2 (Cesari and Kannan [5], Cesari and Pucci [6]). For selfadjoint elliptic problems, Landesman and Lazer [9] proved, also by the alternative method, a remarkable theorem which was then extended by Williams [14] by the same method, and by others by different arguments. Later, Shaw [12] proved, again by the alternative method, that Landesman’s and Lazer’s theorem extends even to nonselfadjoint problems with equal Fredholm indices and whose eigenfunctions share regions of positivity and negativity with their corresponding adjoint eigenfunctions. In the present paper we definitely aim at elliptic problems which are not necessarily selfadjoint and do not necessarily satisfy Shaw’s requirements. The sufficient conditions we obtain are more quantitative in character and concern the cases Nx = f(t) + g(t, D”x) and NX = f(f) + g(t, x(t)), t E G. However, as we show by examples for the case Nx = f(t) + g(t, x(t)), our sufficient conditions for existence allow a great freedom on g, on which no monotonicity is required. * This work was done within the Gruppo Nazionale per I’Analisi Funzionale e le sue Applicazioni de1 Consiglio Nazionale delle Ricerche, Rome, Italy. 1227 1228 L. CESARIand P. PUCCI 2. GENERAL ASSUMPTIONS Let G be a bounded connected open set in [w”with smooth boundary aG: in fact most of our results extend to a compact connected smooth Riemannian n-manifold with or without boundary. On the other hand, we shall consider situations where the smoothness assumptions on aG can be relaxed so as to allow, say, G to be an interval in UP. For s a nonnegative integer we denote by H” the Sobolev (Hilbert) space lV.‘(G) of square integrable functions on G whose (distributional) derivatives of orde,r* ss are also square integrable functions (Ho = L,(G)), with norm &&I = (o~~~J IID”~l~i) ’ . Here we denote by (x, y) and lixllz= (x, x)l’* t h e inner product and norm in L,(G). Then H”, is the linear subspace of H” which is obtained by completion in the norm above of the set of the functions of class c^ and compact support in G. Let E be a (not necessarily selfadjoint) uniformly elliptic real linear partial differential operator of order 2m, i.e. where (Y = (LYE, . cx,,), /3 = (PI. ., /3J, /aI = ml + . + m,,, all cu,, bs are nonnegative integers, with associated linear homogeneous boundary conditions, say Bx = 0 on aG; or more explicitly B,(t, D”x) = 0, j = 1, . , M, where D’x denotes the set of the derivatives D^x on the boundary aG. 0 c I aI s K =Zm. - 1 for some K (the derivatives being replaced by their traces for x in a Sobolev space H”o). We denote by A[@, IQ]the usual bilinear form associated to E: We denote by E* the formal adjoint operator, E*y = .,,F m (-l>‘B’D”(aga(t)D”y), s with the associated boundary conditions, say Dy = 0 on aG. or D,(t, Dfiy) = 0. s = 1. ., N. We assume that both Ker E and Ker E* are finite dimensional, with x > p 3 q 3 0, p = dim Ker E. q = dim Ker E*. (However, we shall consider situations where x = p > q 2 0). A few words on the concept of solution x of the linear problem Ex = f, f E L?(G), with Bix = 0, j = 1. , M. For weak solutions we need only to assume that the coefficients U,~ are measurable bounded functions on G. For instance, for the typical homogeneous Dirichlet boundary problem the boundary conditions are given by x = 0, ax/an = 0, . , d”-lx/dnm-l = 0. n the exterior normal to aG, and we say that x is a weak solution of Ex = fprovided x E H$’ and A[x, ~1 = cf, v) for all v E C;(G). In other words. W = D(E) = Hz is the domain of E. We refer to [2] for the concept of weak solution for more general boundary value problems. For strong, or classical solutions, we need to assume that the coefficients a,@ are of class Ci@l(G). Again, for the homogeneous Dirichlet boundary value problem above we say that x Existence theorems 1229 is a strong solution of Ex = f provided x E HT II HL” and Ex = f a.e. in G. Thus, W = D(E) = Hy n Hz” and W is equipped with the topology of H’“, W = D(E). In general, let W = D(E) be the subspace of all elements x of a Sobolev space H”” for which Bx = 0, and we assume that W is closed in HmO. Since B involves derivatives of orders < K we shall assume K s m. - 1. For weak solutions of the Dirichlet problem above we have K = m - 1, m. = m. For strong solutions of the same problem we have K = m - 1. m. = 2m. We assume that a Fredholm’s alternative theorem holds, that is, for f E Lz(G). the problem EX = f, with BjX = 0, ; = 1, . , M, has a solution x E W if and only if f is orthogonal (in L,(G)) to all elements w of Ker E*. Let X = Y = L,(G). We assume that E is a closed operator with range R(E) closed in Y. All the assumptions above are generally true for reasonable boundary conditions B (cf., e.g., (10, pp. 148-154, ill-1131). Let N be a (nonlinear) Nemitsky operator of the form Nx = f(f) + g(t, D”x), where f is a given element in L*(G), where D”x denotes the set of all derivatives of x of orders LY,0 c /(YI < k,,, where k. is any integer S2m and in any case k. s mo, and where g(t. u). or g: G X WP + R, is measurable in t for all u E [WV,and continuous in u for a.a. I E G. Here ,U = 1 + n + n(n + 1)/2 + . 5 n(n + k. - 1)/2 is the number of different derivatives in R” of orders LY, 0 =Z IcyI s ko. Thus, for g bounded in G X R”, then since 0 =Z k. 6 mo, and W C H”‘o C H’o, then N: W-, @ is a bounded continuous operator from W C H”o to fl = L,(G) in the topologies of H”o and L2(G) (cf. [13, p. 155; 8, p. 271). Again, for g bounded, the range R(N) of N is made up of functions on G which are in absolute value < if(t)1+ sup/gl. If both f and g are bounded, then the range of N is a subset of L,(G), and N is a bounded continuous operator from W to L,(G). If g = g(t, u) is bounded in G X lV and uniformly Lipschitzian in U. then N: W + Ho is a Lipschitzian operator. With these assumptions, we shall consider elliptic problems of the form: Ex = f(t) + g(t, D&X), [EC, (1) Bj(t,D”x)=O onaG, j=l,..., M. 1 3. SOME FURTHER PROPERTIES OF E Let @,, . ., Qp be an orthonormal basis for Ker E, the elements I$, being certainly in X = Lz(G), and let P: X --, X denote the orthogonal projection of X onto X0 = Ker E = = (6, . .I @,)defined by Px = $,(x, $4 @,f or x E X. Then PP = P and we take X, (Ker E)’ = (I - P)X so that we have the decomposition X = X0 + X,, X0 = Ker E. The restriction of E: D(E) C X-, Y to the subspace D(E) n (Ker E)’ is a 1 - 1 closed linear operator, whose closed range is the complete range of E. Hence, by the closed graph theorem, the inverse map H = [EID(E) II (Ker E)‘-I-’ is a 1 - 1 continuous linear operator with domain R(E) and range D(E) n (Ker E)I, or D(H) = R(E), Z?(H) = D(E) fl (Ker E)I. 1230 L. CESARI and P. PUCCI Moreover, EHy=y for ally E R(E), HEX = x - f, (x, c$~)c#J~for allx E D(E), (2) i=l llH~liwc Li!h for ally E Z?(E) and some constant L.