-Tr4j0(.= On the Kolmogorov-Stein Inequality* C,nllfllnoo-llf<nll), IIf()llo _< gn_ / g(n n-k)

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1 J. oflnequal. & Appl., 1999, Vol. 3, pp Reprints available directly from the publisher Photocopying permitted by license only (C) 1999 OPA (Overseas Publishers Association) N.V. Published by license under the Gordon and Breach Science Publishers imprint. Printed in Malaysia. On the Kolmogorov-Stein Inequality* HA HUY BANG a,t and HOANG MAI LE b a lnstitute of Mathematics, RO. Box 631, Bo Ho, Hanoi, Vietnam; b Thai Nguyen Provincial Pedagogic Secondary School, Vietnam Dedicated to Ju. A. Dubinskii on the occasion of his sixtieth birthday (Received 15 December 1997," Revised 6 February 1998) In this paper, we prove the Kolmogorov-Stein inequality for norms generated by concave functions (with the same constants). Keywords." Kolmogorov s inequality; Inequality for derivatives; Theory of Orlicz spaces AMS 1991 Subject Classification." 26B35; 26D10 1, INTRODUCTION Kolmogorov [1] has given the following result: Letf(x),f (x),...,f(n)(x) be continuous and bounded on IR. Then IIf()llo _< C,nllfllnoo-llf<nll), where 0 < k < n, Ck,n gn_ / g(n n-k) Ki -Tr4j0(.= )J/(2j-Jr- l)i+l Supported by the National Basic Research Program in Natural Science and by the NCST "Applied Mathematics." Corresponding author. 153

2 154 H.H. BANG AND H.M. LE for even i, while Ki 1/(2j 4j + )i+1 for odd i. Moreover the constants are best possible. This result has been extended by Stein [2] to Lp-norm and by Ha Huy Bang [3] to any Orlicz norm. The Kolmogorov-Stein inequality and its variants are a problem of interest for many mathematicians and have various applications (see, for example [4,5] and their references). In this paper, modifying the methods of [2,3] we prove this inequality for another norm generated by concave functions. Note that the Orlicz norm is generated by convex functions and here we must overcome some essential difficulties because of the difference between the convex and concave functions. 2. RESULTS Let Z denote the family of all non-zero concave functions (t):[0, o) [0, o], which are non-decreasing and satisfy (0)=0. For an arbitrary measurable function f, E then we define IIIIIN (I) (Af(y))dy, where Af(y) mes{x: If(x)l > y}, (y _> 0). If the space N N(]R) consists of measurable functionsf(x) such that IlfllN < o then N is a Banach space. Denote by M=M(]R), the space of measurable functions g(x) such that I[gll sup (I)(mes A) Then M is a Banach space, too [6,7]. We have the following results [6]: Ig(x)l dx: A C, 0 < mes A < o < o. LEMMA If E, there is an isometric order-preserving isomorphism J: M N, (ofm onto N,) such that J(g)(f) f(x)g(x) dx, (f N, g M).

3 ON THE KOLMOGOROV-STEIN INEQUALITY 155 LEMMA 2 IffE Ne, g Me then fg L1 and Now, we give the main theorem: If(x)g(x)l dx <-IIf[[N[[gllM", THZOREM Let f(x) and its generalized derivative f(n)(x) be in Ne. Then f(k)(x) Ne for all 0 < k < n and IIf(k)[[% < Ck,nl[fl[nN-k[[f(n)[[ k N" (1) Proof 0<k<n. then We begin to prove (1) with the assumption that f(k)(x) Ne, By virtue of Lemma 1, it is clear thatn Me, and iffe Ne, g f, g) J(g)(f) f(x)g(x) dx. Therefore, since Ilxllx-Ilxll** for any normed space X [9, p. 1131, we have Ilf(llN sup I(f(,g)l Ilgllt--1 Put Me sup f(k)(x)g(x)dx. (2) Let e > 0. We choose a function h(x) Me such that IlhllMo and f(k)(x)h(x) dx >_ IIf(llNo. (3) F(x) f (x + y)h(y) dy. By Lemma 2 IF(x)l f(x + y)h(y) dy <_ If(x + y)h(y)l dy <-- [If(x + ")[INI[h[[M --IlfllN,

4 156 H.H. BANG AND H.M. LE where the last equality holds because of (2) and the definition Then F(x)E Lo(R), and arguing as in [3] we get in the distribution sense. For all x E R, clearly F (r) (x) f(r) (x + y)h(y)dy, 0 <_ r < n (4) IF(r)(x)l IIf(r)(x + ")IINIIhlIM -IIf(r)llN Now we prove continuity of F(r)(x) on IR (0 _< r _< n). We show this for r-0 by contradiction: Assume that for some e>0, point x and subsequence Itkl 0 (f(x + tk + y) --f(x + y))h(y)dy _>e, k>_ 1. (5) Since fn we get easily fll,loc(][{). Then for any m= 1,2,...,f(tk+y)f(y) in Ll(-m,m). Therefore, there exists a subsequence, denoted again by {tk}, such that f(tk+y)f(y) a.e. in (--re, m). Therefore, there exists a subsequence (for simplicity of notation we assume that it is coincident with {tk}) such that f(x + tk + y)--,f(x + y) a.e. in (--oe, On the other hand, {f(x + tk + y)} is bounded in Ne because of IIf(x + tk + ")IIN --[IfllN, k >_ 1. So {f(x+ tk +y)} is a weak precompact sequence. Therefore, there exist a subsequence denoted by {f(x+tk+y)} and a function f, (y) 6 Ne such that f(x when k oe, Vv(y) N. It means -- + tk + y), v(y)) (f,(y), v(y)) f (x + tk + y)v( y) dy f,( y)v( y) d(y), Vv(y) Me. (6)

5 ON THE KOLMOGOROV-STEIN INEQUALITY 157 Let u(x) be an arbitrary function in C (R), then u(x) E M. Therefore, by (6) we get f(x + tk + y)u(y) dy --+ f,(y)u(y) dy, w e cg(a). Because each u E C(R) has a finite support, then it follows from f(x + t + y) f(x + y) a.e. that f(x + & + y)u( y) dy --+ f(x + y)u(y) dy, Vu cg(). Combining (6), (7), we have f(x + y)u(y) dy f,(y)u(y) d(y), Vu Then it is known that [8, p. 15]: Therefore, f(x + y) f, y) a.e. f (x + & + y)h(y) dy f(x + y)h(y) dy because of (6), which contradicts (5). The cases < r < n are proved similarly. The continuity of F(r)(x) has thus been proved. The functions F(O(x) are continuous and bounded on R. Therefore, it follows from the Kolmogorov inequality and (3), (4) that (7) e)n _< IF()(O)I,, < IIF()II (8) On the other hand, IlFIIoo <_ IIf(x + Y)IINIIh(Y)IIM --IlfliN, IlF( )lloo <- Ilf( )(x + Y)IINIIh(Y)IIM IIf( )tln (9) (0)

6 158 H.H. BANG AND H.M. LE Combining (8)-(10), we get (llf()lln_ f.)n _< Ck,nllflln-kN [If()llN" By letting e 0 we have (1). To complete the proof, it remains to show that f(k) E Ne, 0 < k < n iff, f(n) N. Let a(x)c(r),. ba(x)_>0, ba(x)=0 for Ixl_> and fa(x)dx= 1. We put fa=f*2a. Then fac(r) because of f Ll,loc(R). Therefore,f (k) =f* b(), k > 0 and it is easy to check that f(n) f(n), Now we provef () f,() Ne, k >_ 0. Actually, for k 0 it follows that sup Ilgllg----1 (f * 2),)(x)g(x) dx sup f(x y)b),(y) dy g(x)dx Ilgll= sup f(x y)g(x) dx (y)dy Ilgll=l sup f(x y)g(x) dx I(y)l dy IlgllM=l sup IlgllM=l II/("- Y)IINI(Y)I dy sup [IfllN Ia(y) Idy Ilgllg=l Ilflllllll. The cases k > 0 are proved similarly. Therefore, by the fact proved above, we have Therefore, since

7 and ON THE KOLMOGOROV-STEIN INEQUALITY 159 [If(.xn)l[N <_ IIf(n)[[N[l,Xlll I[f(n)[[N, we get that, for any 0 < k <_ n, the sequence {f(k)} is bounded in Now we prove that, for any 0 < k _< n, there exists a subsequence, which is weakly convergent to some gk 6 Ne. We will show, for example, the fact thatf is weakly convergent to fby contradiction: Assume that for some e0 > 0, g Me and a subsequence Ak 0, (f k (x) f(x) )g(x) dx > co, k>l. (11) Then, it is known thatfa f, A 0 in Ll,loc(R). Therefore, there exists a subsequence {km} (for simplicity we assume that km m) such that f (x) f (x) a.e. On the other hand, {fak} is bounded in Ne because of II/ IINo --< IlfllN" So {fa} is a weak precompact sequence. Therefore, there exists a subsequence, denoted again by {fa}, and a functionf. (x) Ne such that f(x)v(x)dx f,(x)v(x)d(x), Vv(x) Me. (12) By an argument similar to the previous one, we get Therefore, f(x) f, (x) a.e. f (x)v(x) dx f (x)v(x) dx because of (12), which contradicts (11). Finally, it follows from weak convergence faf that for any v (f})(x), p(x)) (--1)k(fa(x)p(k)(X)) (--1)(f(x), p(k)(x)) (f()(x), p(x)).

8 160 H.H. BANG AND H.M. LE Therefore, since the weak convergence of some subsequence of {f(g) } to gk E N, we get f(k) gg Ne (0 < k < n). So we have proved the fact that f(g) N for all 0 < k < n iff, f(n) N,. The proof is complete. Remark For periodic functions we have: THEOREM 2 Let (t) _., f(x) and its generalized derivative f(n)(x) be in N(q). Then f(k)(x) N(q) for all 0 < k < n and If() IIv() < C,n Ifl n-g() If (n) III where q is the torus and II1.111/the corresponding norm. References 1] A.N. Kolmogorov, On inequalities between upper bounds of the successive derivatives of an arbitrary function on an infinite interval, Amer. Math. Soc. Trans. Ser. 1, 2, 1962, [2] E.M. Stein, Functions of exponential type, Ann. Math., 65 (1957), [3] Ha Huy Bang, A remark on the Kolmogorov-Stein inequality, J. Math. Anal. Appl., 203 (1996), [4] M.W. Certain and T.G. Kurtz, Landau-Kolmogorov inequalities for semigroups and groups, Proc. Amer. Math. Soc., 63 (1977), [5] V.M. Tikhomirov and G.G. Magaril-II jaev, Inequalities for derivatives, in "Kolmogorov A. N. Selected Papers", Nauka, Moscow, 1985, pp [6] M.S. Steigerwalt and A.J. White, Some function spaces related to Lp, Proc. London. Math. Soc., 22 (1971), [7] M.M. Rao and Z.D. Ren, Theory of Orlicz spaces, Marcel Dekker Inc., New York, [8] L. H6rmander, The Analysis of Linear Partial Differential Operators L Springer- Verlag, Berlin, Heidelberg, [9] K. Yosida, Functional Analysis, Springer-Verlag, New York, 4th edn., 1974.