POSITIVE SOLUTIONS OF A NONLINEAR THREE-POINT BOUNDARY-VALUE PROBLEM. Ruyun Ma

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1 Electronic Journal of Differential Equations, Vol. 1998(1998), No. 34, pp ISSN: URL: or ftp ejde.math.swt.edu (login: ftp) POSITIVE SOLUTIONS OF A NONLINEAR THREE-POINT BOUNDARY-VALUE PROBLEM Ruyun Ma Abstract. We study the existence of positive solutions to the boundary-value problem u + a(t)f(u) =, t (, 1) u() =, αu()=u(1), where <<1and<α<1/. We show the existence of at least one positive solution if f is either superlinear or sublinear by applying the fixed point theorem in cones. 1. Introduction The study of multi-point boundary-value problems for linear second order ordinary differential equations was initiated by Il in and Moiseev [7, 8]. Then Gupta [5] studied three-point boundary-value problems for nonlinear ordinary differential equations. Since then, the more general nonlinear multi-point boundary value problems have been studied by several authors by using the Leray-Schauder Continuation Theorem, Nonlinear Alternatives of Leray-Schauder, and coincidence degree theory. We refer the reader to [1-3, 6, 1-12] for some recent results of nonlinear multi-point boundary value problems. In this paper, we consider the existence of positive solutions to the equation with the boundary condition u + a(t)f(u) =, t (, 1) (1.1) u() =, αu()=u(1), (1.2) where <<1. Our purpose here is to give some existence results for positive solutions to (1.1)-(1.2), assuming that α < 1 and f is either superlinear or sublinear. Our proof is based upon the fixed point theorem in a cone. From now on, we assume the following: (A1) f C([, ), [, )); (A2) a C([, 1], [, )) and there exists x [, 1] such that a(x ) > 1991 Mathematics Subject Classifications: 34B15. Key words and phrases: Second-order multi-point BVP, positive solution, cone, fixed point. c 1999 Southwest Texas State University and University of North Texas. Submitted June 9, Published September 15, Partially supported by NNSF (198128) of China and NSF (ZR-96-17) of Gansu Province 1

2 2 Ruyun Ma EJDE 1999/34 Set f(u) f = lim u + u, f f(u) = lim u u Then f =andf = correspond to the superlinear case, and f = and f = correspond to the sublinear case. By the positive solution of (1.1)-(1.2) we understand a function u(t) whichispositiveon<t<1andsatisfies the differential equation (1.1) and the boundary conditions (1.2). The main result of this paper is the following Theorem 1. Assume (A1) and (A2) hold. Then the problem (1.1)-(1.2) has at least one positive solution in the case (i) f =and f = (superlinear) or (ii) f = and f =(sublinear). The proof of above theorem is based upon an application of the following wellknown Guo s fixed point theorem [4]. Theorem 2. Let E be a Banach space, and let K E be a cone. Assume Ω 1, Ω 2 are open subsets of E with Ω 1, Ω 1 Ω 2,andlet A:K (Ω 2 \Ω 1 ) K be a completely continuous operator such that (i) Au u, u K Ω 1,and Au u, u K Ω 2 ;or (ii) Au u, u K Ω 1,and Au u, u K Ω 2. Then A has a fixed point in K (Ω 2 \ Ω 1 ). 2. The Preliminary Lemmas Lemma 1. Let α 1then for y C[, 1], the problem has a unique solution t u(t) = (t s)y(s)ds αt The proof of this lemma can be found in [6]. u + y(t) =, t (, 1) (2.1) u() =, αu()=u(1) (2.2) ( s)y(s)ds + t (1 s)y(s) ds Lemma 2. Let <α< 1.Ify C[, 1] and y, then the unique solution u of the problem (2.1)-(2.2) satisfies u, t [, 1] Proof From the fact that u (x) = y(x), we know that the graph of u(t) is concave down on (,1). So, if u(1), then the concavity of u and the boundary condition u() = imply that u fort [, 1]. If u(1) <, then we have that u() < (2.3) and u(1) = αu() > 1 u(). (2.4) This contradicts the concavity of u.

3 EJDE 1999/34 Positive solutions 3 Lemma 3. Let α > 1. Ify C[, 1] and y(t) for t (, 1), then (2.1)-(2.2) has no positive solution. Proof Assume that (2.1)-(2.2) has a positive solution u. If u(1) >, then u() > and u(1) 1 = αu() 1 > u() (2.5) this contradicts the concavity of u. If u(1) = and u(τ) > forsomeτ (, 1), then u() = u(1) =, τ (2.6) If τ (,), then u(τ) >u()=u(1), which contradicts the concavity of u. If τ (, 1), then u() = u() <u(τ) which contradicts the concavity of u again. In the rest of the paper, we assume that α < 1. Moreover, we will work in the Banach space C[, 1], and only the sup norm is used. Lemma 4. Let <α< 1.Ify C[, 1] and y, then the unique solution u of the problem (2.1)-(2.2) satisfies where γ =min{α, α(1 ) 1 α,}. inf u(t) γ u, t [,1] Proof. We divide the proof into two steps. Step 1. We deal with the case <α<1. In this case, by Lemma 2, we know that u() u(1). (2.7) Set If t <1, then and u( t) u(1) + u( t) = u. (2.8) min u(t) =u(1) (2.9) t [,1] u(1) u() ( 1) 1 = u(1)[1 1 1 α 1 ] = u(1) α(1 ). This together with (2.9) implies that min t [,1] α(1 ) u(t) u. (2.1)

4 4 Ruyun Ma EJDE 1999/34 If < t<1, then min u(t) =u(1) (2.11) t [,1] From the concavity of u, we know that u() u( t) t Combining (2.12) and boundary condition αu() = u(1), we conclude that This is u(1) α u( t) t u( t) = u. (2.12) min u(t) α u. (2.13) t [,1] Set Step 2. We deal with the case 1 α< 1. In this case, we have u() u(1). (2.14) u( t) = u (2.15) then we can choose t such that t 1 (2.16) (wenotethatif t [, 1] \ [, 1], then the point (, u()) is below the straight line determined by (1,u(1)) and ( t, u( t)). This contradicts the concavity of u. From (2.14) and the concavity of u, we know that min u(t) =u(). (2.17) t [,1] Using the concavity of u and Lemma 2, we have that This implies This completes the proof. u() u( t). (2.18) t min u(t) u. (2.19) t [,1] 3 Proof of main theorem Proof of Theorem 1. Superlinear case. Suppose then that f =andf =. We wish to show the existence of a positive solution of (1.1)-(1.2). Now (1.1)-(1.2) has a solution y = y(t) if and only if y solves the operator equation y(t) = + t (t s)a(s)f(y(s))ds t def = Ay(t). αt ( s)a(s)f(y(s))ds (3.1)

5 EJDE 1999/34 Positive solutions 5 Denote K = {y y C[, 1],y, min y(t) γ y }. (3.2) t1 It is obvious that K is a cone in C[, 1]. Moreover, by Lemma 4, AK K. It is also easy to check that A : K K is completely continuous. Now since f =,wemaychooseh 1 >sothatf(y)ɛy, for<y<h 1, where ɛ>satisfies ɛ 1 α (1 s)a(s)ds 1. (3.3) Thus, if y K and y = H 1, then from (3.1) and (3.3), we get Ay(t) t t ɛ ɛ (1 s)a(s)ɛy(s)ds (1 s)a(s)ds y (1 s)a(s)dsh 1. (3.4) Now if we let Ω 1 = {y C[, 1] y <H 1 }, (3.5) then (3.4) shows that Ay y,fory K Ω 1. Further, since f =, there exists Ĥ2 > such that f(u) ρu, for u Ĥ2, where ρ> is chosen so that ρ γ 1 α (1 s)a(s)ds 1. (3.6) Let H 2 =max{2h 1, Ĥ2 γ }and Ω 2 = {y C[, 1] y <H 2 },theny Kand y = H 2 implies min y(t) γ y Ĥ2, t1

6 6 Ruyun Ma EJDE 1999/34 and so Ay() = + = 1 = 1 + = ( s)a(s)f(y(s))dt α ( s)a(s)f(y(s))ds ( s)a(s)f(y(s))ds + a(s)f(y(s))ds + 1 sa(s)f(y(s))ds a(s)f(y(s))ds sa(s)f(y(s))ds (3.7) a(s)f(y(s))ds + 1 sa(s)f(y(s))ds sa(s)f(y(s))ds a(s)f(y(s))ds sa(s)f(y(s))ds (by <1) =. Hence, for y K Ω 2, Ay ρ γ (1 s)a(s)ds y y. Therefore, by the first part of the Fixed Point Theorem, it follows that A has afixedpointink (Ω 2 \Ω 1 ), such that H 1 u H 2. This completes the superlinear part of the theorem. Sublinear case. Suppose next that f = and f =. WefirstchooseH 3 > such that f(y) My for <y<h 3,where 1 Mγ( 1 α ) (1 s)a(s)ds 1. (3.8) By using the method to get (3.7), we can get that Ay() = ( s)a(s)f(y(s))dt α ( s)a(s)f(y(s))ds + (3.9) (1 s)a(s)my(s)ds H 3 (1 s)a(s)mγds y

7 EJDE 1999/34 Positive solutions 7 Thus,we may let Ω 3 = {y C[, 1] y <H 3 }so that Ay y, y K Ω 3. Now,since f =, there exists Ĥ4 > sothatf(y)λy for y Ĥ4,where λ>satisfies λ 1 1 α [ (1 s)a(s)ds] 1. (3.1) We consider two cases: Case (i). Suppose f is bounded, say f(y) N for all y [, ). In this case choose N 1 H 4 =max{2h 3, (1 s)a(s)ds} 1 α so that for y K with y = H 4 we have t Ay(t) = (t s)a(s)f(y(s))ds αt ( s)a(s)f(y(s))ds t 1 + t 1 (1 s)a(s)nds H 4 and therefore Ay y. Case (ii). If f is unbounded, then we know from (A1) that there is H 4 : H 4 > max{2h 3, 1 γ Ĥ4} such that f(y) f(h 4 ) for <yh 4. (We are able to do this since f is unbounded). Then for y K and y = H 4 we have t Ay(t) = (t s)a(s)f(y(s))ds αt ( s)a(s)f(y(s))ds t 1 + t (1 s)a(s)f(h 4 )ds 1 (1 s)a(s)λh 4 ds H 4. Therefore, in either case we may put Ω 4 = {y C[, 1] y <H 4 }, and for y K Ω 4 we may have Ay y. By the second part of the Fixed Point Theorem, it follows that BVP (1.1)-(1.2) has a positive solution. Therefore, we have completed the proof of Theorem 1.

8 8 Ruyun Ma EJDE 1999/34 REFERENCES [1] W. Feng and J. R. L. Webb, Solvability of a three-point nonlinear boundary value problems at resonance, Nonlinear Analysis TMA, 3(6), (1997), [2] W. Feng and J. R. L. Webb, Solvability of a m-point boundary value problems with nonlinear growth, J. Math. Anal. Appl., 212, (1997), [3] W. Feng, On a m-point nonlinear boundary value problem, Nonlinear Analysis TMA 3(6), (1997), [4] D. Guo and V. Lakshmikantham, Nonlinear Problems in Abstract Cones, Academic Press, San Diego [5] C. P. Gupta, Solvability of a three-point nonlinear boundary value problem for a second order ordinary differential equation, J. Math. Anal. Appl. 168, (1992), [6] C. P. Gupta, A sharper condition for the solvability of a three-point second order boundary value problem, J. Math. Anal. Appl., 25, (1997), [7] V. A. Il in and E. I. Moiseev, Nonlocal boundary value problem of the first kind for a Sturm-Liouville operator in its differential and finite difference aspects, Differential Equations 23, No. 7 (1987), [8] V. A. Il in and E. I. Moiseev, Nonlocal boundary value problem of the second kind for a Sturm-Liouville operator, Differential Equations 23, No. 8 (1987), [9] M. A. Krasnoselskii, Positive solutions of operator equations, Noordhoff Groningen, (1964). [1] S. A. Marano, A remark on a second order three-point boundary value problem, J. Math. Anal. Appl., 183, (1994), [11] R. Ma, Existence theorems for a second order three-point boundary value problem, J. Math. Anal. Appl. 212, (1997), [12] R. Ma, Existence theorems for a second order m-point boundary value problem, J. Math. Anal. Appl. 211, (1997), [13] R. Ma and H. Wang, On the existence of positive solutions of fourth-order ordinary differential equations, Appl. Analysis, 59, (1995), [14] H. Wang, On the existence of positive solutions for semilinear elliptic equations in annulus, J.Differential Equations 19, (1994), 1-7. [15] C. Gupta and S. Trofimchuk, Existence of a solution to a three-point boundary values problem and the spectral radius of a related linear operator, Nonlinear Analysis TMA, 34, (1998), Ruyun Ma Department of Mathematics, Northwest Normal University Lanzhou 737, Gansu, P. R. China address: mary@nwnu.edu.cn