# Czechoslovak Mathematical Journal

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1 Czechoslovak Mathematical Journal Garyfalos Papaschinopoulos; John Schinas Criteria for an exponential dichotomy of difference equations Czechoslovak Mathematical Journal, Vol. 35 (1985), No. 2, Persistent URL: Terms of use: Institute of Mathematics AS CR, 1985 Institute of Mathematics of the Czech Academy of Sciences provides access to digitized documents strictly for personal use. Each copy of any part of this document must contain these Terms of use. This document has been digitized, optimized for electronic delivery and stamped with digital signature within the project DML-CZ: The Czech Digital Mathematics Library

2 Czechoslovak Mathematical Journal, 35 (110) 1985, Praha CRITERIA FOR AN EXPONENTIAL DICHOTOMY OF DIFFERENCE EQUATIONS GARYFALOS PAPASHINOPOULOS and JOHN SCHINAS, Xanthi (Received December 19, 1983) In this paper we give sufficient and necessary conditions for exponential dichotomy of a linear difference equation having the form (1) x{n + 1) = Ä{n) x{n), where Ä{n) is a fc x /с invertible matrix for n en such that (2) \Ä{n)\SM for n = 1, 2, 3,..., M^l with elements ^^/(w) real functions on iv = {0, 1,...}. In what follows we denote by I I any convenient norm either of a vector or of a matrix. The difference equation (1) is said to possess an exponential dichotomy on the set N if there exist a projection P, that is a matrix such that P^ = P and constants ^>0, 0<p<l such that (3) \X{n) P\ й Xi?""'" X(m) P, n ^m^o, \X{n) (/ - P)\ й K/"~'' X(m) (/ - P), m ^ n ^ 0, where X{n) is the matrix solution X(n) = A{n 1)... A(o), X{o) = /. Since A(n) is a /c Xfcinvertible and bounded matrix, it can be easily proved that this definition is equivalent to Henry's definition [3, p. 229]. First we prove a lemma which we use in the following. Lemma 1. Suppose that (1) has exponential dichotomy for n ^ T, TeN. Then (1) has exponential dichotomy for n ^ 0. Proof. From (2) we have (4) Z(n)(/-P) ^M" (/-P)l. Since (1) has exponential dichotomy for m ^ Twe have (5) \X{T){I-P)\йKp^^^-'^\Xim){I-P)\. 295

3 Let О S n й Т ^ т. Then, by (4), (5), we have \X{n){l - P)\ < M"\{I - P)\ S ущ^-~-^\~ \X(T)(I - P)\ < < \x{r){i - p) ' ^ ^^ ^' Let 0 й n S m S T and Ki = min {[^(n) (/ ~ P)\: 0 ^ n g Г}. We \X{n) {I - P) Ф 0 for every nen. So K^ = 0. Therefore have \X{n){I - P)\ S M%I - P)\ й K^'M^^ - P)\ \X{m)(l - P)\. Since 0 g n g m ^ Twe have 1 g ^-r+m-«^^^ therefore \X{n) {I - P)\ й К-, \Mp-'Y (/ - P)\ р^-щш) (/ -- P)\. In the same manner one can prove the first inequality of the exponential dichotomy. We prove now our main results. The following two propositions are the discrete analogue of those which have been proved by Goppel [2, p. 14] in the continuous case. The adaptation from the continuous to the discrete case is not direct but requires some special devises. Proposition 1. Suppose that (1) has exponential dichotomy. Then there exist constants 0 < Ö < 1, T > 0, TeN such that Proof. We set Then \x{n)\ s 0 sup {\x{u)\ :\u ~ п\й T, u.nen, n ^ T}. x,{n) = X{n) p^, X2{n) = X{n) (/ - P) ^ x{n) = Xi(n) + X2{n). First consider the case :'C2(^) ^ ^:i(m), for some m e N. From (3), for n ^ m ^ 0, we have \x2{m)\ ^ Kp^-^\x2{n)\ or \x2{n)\ ^ К-'р~^"-^Щт)\ Therefore x,(n)l gx/-1xi(m)l or ~\x,{n)\^ -Kf~-\x,{m)\. \x{n)\ = \x,{n) + X2{n)\ à 1^2(n)l _ \x,{n)\ ^ K-'p-^"-^'\x2{m)\ - Kp^-"^\x,{m)\, \x{n)\ ^ (K" V~~^"~^) ~- Xp"-"^) lx2(m)l, n-^m^o or (6) \x{n)\ ^ ^(Х"^Р^Сп-т) _ Kf-"") \x{m)\, n^m^o Now consider the case \xn (m)i S xi(m), for some men. Similarly, we get (7) \x{n)\ ^ ^{K-'^P" i)-»-" - Kp""-") \x{m)\, m^n^o. 296

4 We choose TeN sufficiently large and 0 < Ö < 1 so that From (6) and (7) we obtain that \x{m)\ ^ в sup {\x{n)\ :\m - n\ ^ T, n, m e N, m ^ T]. Next we prove a proposition, which is actually the converse of Proposition 1. Proposition 2. Suppose that there exist constants T^ 1, TEN, and 0 < Ö < 1 such that (8) \x{n)\ S 0 sup {\x{u)\: \u - n\ й T, u, n en, n '^ T}. Then (1) has an exponential dichotomy. Proof. Let и be the set of such и e R^ that the solution x of (1) fulfilling x(0) = и is bounded. Obviously, t/ is a linear space. Let X be a solution of (l) with x(0) e U. Since a contradiction with (8) results from Hm sup \x[n)\ > 0, we have n-^ 00 lim x(/i) = 0. П-* CX) For any men we conclude again by (8) that max { x(m), x(m + l),..., \x{m + T l) } = max { х(п) : n = /ti, m + 1,...},. \x[n)\ ^ 0 max { x(m), x(m + 1),..., x(m + T 1) } for n = m + T, m + T+ 1, m + Т+ 2,..., and by induction (9) \x{n)\ ^ вчъ^х {\x{m)l \x{m + l),..., \x{m + T- l) } for n = m + kz m + кт+ 1, m + /сг 4-2,..., к = I, 2, 3,.... By (2) and (9) we have (10) \x{n)\ S X/~'" x(m) for n^ m^o with p = Q^'^, К = М^~Ч~^. Let X be a solution of (1) with x(0) e R^\U. Since x is unbounded, there exists such an s(x(0)) e N (we shall write s instead of 5(x(0))) that x(5) ^ M^ x(0), x(n) < < M'^ x(0) for П = 0, 1,..., 5 - L By (2) we have s ^ T and (8) implies that a sequence of integers t^, ^2, ^з, exists such that t^ ~ s, t^ < ti+^ ^ /, + T, \x[tj+^)\ ^ ^ ö"^ x(;,-), \x(n)\ < e-'\x{ti)\ for f,- ^ П < r,+ i, / = 1, 2, 3,.... Let s -^ n < m. Find ijen such that r^- ^ n < r^ + i, ^j_i < m ^ f^. Then we have x(m) ^ M-^^^ x(r,.) ^ M-^-^^ö-^'^' x;^-) ^ M-^^^"^^0-^'^' x(n). Since (j /) T ^ m -- /7, we have (11) \x(n)\ й X/'"i^(m) ^ith p = Q^i^, К = M^^^-^>. 297

5 Let F be a complementary space to U in R^ (i.e. R^ - = U + V). Put 5 = sup {s{v): VEV\{0}}, Since S = sup {s{v): v e V, \v\ = l}, we obtain by a compactness argument that S < oo. Let F be the projection on U along V. Then from (10), (11) the dichotomy (3) holds for n,m^ S with p = в^'^, К = M^^^~^^ö~^, and by Lemma 1 equation 1 has an exponential dichotomy for n, m ^ 0 (provided that \Н\ = sup { Яз; :}; e R^^ \y\ ^ 1} holds for к X к matrices Я). The proof is completed. We can apply the above propositions to prove the following proposition (cf. Palmer [4, p. 187] for the continuous case). Proposition 3. Suppose that Ä{n) is a к x к bounded upper triangular and invertible matrix for all nen. Then (1) has exponential dichotomy if and only if the corresponding diagonal system (12) x{n + 1) = diag(ai i(n),... а^^(?г)) x{n) has an exponential dichotomy. Proof. Suppose that (12) has an exponential dichotomy and let x(n) = = diag (1, ß, ß^,..., ß^~^) y{n) be a j5 transformation, according to Bylov [1, p. 605]. Let /aii(n) ßa^2{n)... y^''"'ai/c(«)\ (13) у{п-^1) = \ ^ "-^") '" ^"''^Щу{п) öcj,k{n)j From the fact that (12) has an exponential dichotomy and the roughness of the exponential dichotomy [3, p. 232] (13) has an exponential dichotomy provided ß is taken sufficiently small. Since (1) is kinematically similar to (13), also (1) has an exponential dichotomy. Conversely, suppose that (1) has exponential dichotomy. We show, by induction, that (12) has an exponential dichotomy. It is obvious for к = 1. Considering it is true for /c 1 we show that it is true for all ken. Since (l) has an exponential dichotomy according to Proposition 1 there exist T^l, O^0<1 such that for any solution of (1): \x{n)\ й 0 sup {\x{s)\: \s - n\s T}. This is true for all solutions of (1) and, therefore, also for those solutions of (1), which have the last coordinate equal to zero. Hence, by Proposition 2, the equation /aii(n) (x^2{n)... cci,k-i{n) \ 4n+l)=\ ^ "^^W ^^'-^W Un) a,_i,,_i(n)/ has an exponential dichotomy. Therefore the diagonal system x{n + 1) = 298

6 = diag(aii(n),..., %_i/^_i(n)) x(n) has an exponential dichotomy. According to [3, p. 230] the equation x{n + 1) = Ä{n) x{n) + g{n), where Ö^*(W) = [0, 0,...,/(n)] has a bounded solution for every bounded function f{n). So the equation (14) x{n + 1) = a,fc(n) x{n) + f{n) has a bounded solution for every bounded function f(n). Therefore by [3, p. 230] the homogeneous equation of (14) has an exponential dichotomy and the proof is completed. Acknowledgement. We wish to thank Prof. J. Kurzweil for his useful comments and his improvement of the proof of the Prop. 2. References [1] B. F. Bylov: Almost reducible systems, Siberian Math. J. 7 (1966), [2] W. A. Goppel: Dichotomies in Stability Theory, Lecture Notes in Mathematics, No 629, Springer Verlag, Berlin, [3] D. Henry: Geometric Theory of Semilinear Parabolic Equations, Lecture Notes in Mathematics No 840, Springer-Verlag, Berlin, [4] K. J. Palmer: Exponential dichotomy, integral separation and diagonalizability of linear systems of ordinary differential equations. J. Differential Equations 43 (1982), Authors' address: Democritus University of Thrace, School of Engineering, Xanthi, Greece. 299

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