Galois Theory of Difference Equations by Marius van der Put

By Marius van der Put

This publication lays the algebraic foundations of a Galois idea of linear distinction equations and exhibits its courting to the analytic challenge of discovering meromorphic features asymptotic to formal suggestions of distinction equations. Classically, this latter query was once attacked through Birkhoff and Tritzinsky and the current paintings corrects and vastly generalizes their contributions. additionally effects are awarded in regards to the inverse challenge in Galois thought, powerful computation of Galois teams, algebraic houses of sequences, phenomena in confident features, and q-difference equations. The e-book is geared toward complicated graduate researchers and researchers.

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Thus a simple zero is a zero of order 1. Definition 8. Leta E C, r > 0 and let f be holomorphic on {z E qo < Iz - al < r}. We say that a is an essential singularity of f (or that f has an essential singularity at a) if, in the Laurent expansion 00 L n=-oo Cn (z - a)n of f at a, there are infinitely many n < 0 with Cn =1= o. This is equivalent to saying that f is not meromorphic on D(a, r). Theorem 4 (The Casorati-Weierstrass Theorem). Let a E C, r > 0, D* = {z E q Iz - al < r}. Let f E H(D*) and suppose that a is an essential singularity of f· Then f(D*) is dense in C.

Since Cn does not depend on p, we have 00 f(w) = LcnW n, 0< Iwl < r. (D) (Weierstrass' theorem). Clearly FID* = f. Another proof, not using the Laurent expansion, runs as follows. 40 Chapter I. Elementary Theory of Holomorphic Functions If f E H(D*) and zf(z) -+ 0 as z -+ 0, z i= 0, define a function g on D by g(z) = Z2 f (z), z i= 0, g(O) = O. Then g is (:-differentiable at 0 with g' (0) = 0; in fact I -(g(S) - g(O») s = l;f(S) -+ 0 as Since clearly g is (:-differentiable on D*, we have g since g(O) = g'(O) = O.

Let Q be a connected open set in C and let f, g {z E Qlf(z) = g(z)} E 'H(Q). If the set =f. 0 Chapter 1. Elementary Theory of Holomorphic Functions 24 has a point of accumulation in n, then f == g. This is simply Theorem 2 applied to f - g. We now pass on to the maximum principle and the open mapping theorem which are of fundamental importance. We begin with a very simple result which we shall use again later (in Chapter 4). Lemma 2. Let I be an open set in ]R and

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