It is proven in the present article that the solutions of infinite order differential equation with holomorphic parameter w E U C a! depend holornorphically on w in the neighborhood of characterisitic points of certain directions.
Preliminaries and statement of the main theoremsIt was shown in [7, 9, 131 that, roughly speaking, the solutions of the system of differential equations cannot change holomorphically with respect to the parameters w = (wl,. . . , wn), even when the coefficients of the system depend holomorphically on w . In the present note we study this phenomena for infinite order differential equations with holomorphic parameters and determine when such a problem has a solution. More precisely, we construct a fundamental solution, holomorphic in w near characteristic points of certain directions. However a basic gap remains, since the method presented here excludes all the "real" directions when n > 1, [7]. Our approach has its origins in [4]. Let U be a connected open set in C . Here we will consider a special class of operators of infinite order P with holomorphic coefficients. These are operators that can be written in the form P = P ( w , D ) = c a,(w)DQ Q with D = (D1, . . . , D,) = ( G , 8 . . . , &) and a, holomorphic functions on U satisfying the following estimates: For each compact set K C C U and for each h > 0 there 1991 Mathematics Subject Classification. Primary 32A15; Secondary 35R50, Keywords and phmses. Entire functions, characteristic point.As usual a subset R of U x 6" is called conic if for all t > 0, (w,tC) E R whenever (w, C) E R. For simplicity we will assume that an open conic neighborhood of (w, 5)in U x 6" is a set of the form G x V, where G is an open connected subset of U andV is an open, convex cone in C ". w E u, P(w,C) = P W ( 0 . Definition 1.2. ([l, 81.) A point (wg,co) E U x C" is called characteristic for the operator P(w,D,) if for every open conic neighborhood R = G x V of (wo,C0) and for every T > 0 there exists (w',C') E Cl n { (w,C) E U x C" : ICl > r } so that P(w'l<') = 0. Vidras, On Holomorphic Perturbations 269 Since we view the symbol P(w,C) &a a family {Pw(C)},E~ of entire functions of exponential type zero, with holomorphic parameter 20, we have the following equivalent formulation of the Definition 1.2. Definition 1.3. A point (200, ( 0 ) E U x C " is called characteristic for the family of operators {Pw(Dz)}wE~ if for every open neighborhood G, , of W O , for every open, convex cone Vc, containing Co and for any r > 0 there exist w t E G, , and C' in Vc, n (5 E 6" : ICI > r } so that P,t(C') = 0. We denote the set of all characteristic points by CharP. Example 1.4. In the above Example 1.1 the point (0,l) is characteristic for the operator Consider the unit sphere S2"-l = {& C # O , < € a " } .-By adding the above sphere at "infinity" to C " we obtain the compact space 6 ", equiped with the usual topology. To indicate the fact that we consider the unit sphere at infinity we denote it by S2-l and by = the direction of C E C", C # 0. Definition 1.5. We w...