EQUATIONS WITH A PARAMETER



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Journal of Applied Mathematics Stochastic Analysis, 10:3 (1997), 273-278. MONOTONE ITERATIONS FOR DIFFERENTIAL EQUATIONS WITH A PARAMETER TADEUSZ JANKOWSKI Technical University of Gdansk, Department of Numerical Analysis Gdansk, POLAND V. LAKSHMIKANTHAM Florida Institute of Technology, Applied Mathematics Program Melbourne, FL 32901 USA (Received February, 1997; Revised June, 1997) Consider the problem y (t)- f(t,y(t),i), (o)- o, (,) 0. t e J [0, b], Employing the method of upper lower solutions the monotone iterative technique, existence of extremal solutions for the above equation are proved. Key words: Monotone Iterations, Differential Equations, Monotone Iterative Technique. AMS subject classifications: 34A45, 34B99. 1. Prehminaries Consider the following differential equation x (t) = f(t,x(t),), t E J : [O,b] (la) with the boundary conditions x(o)- ko, x(b) kl, (lb) where f C(J R R,R) ko, k 1 R are given. The corresponding solution of (1) yields a pair of (x, ) CI(J, R) x R for which problem (1) is satisfied. Problem (1) is called a problem with a parameter. Conditions on f which guarantee the existence of solutions to (1) are important analysis theorems. Such theorems can be formulated under the assumption that f satisfies the Lipschitz condition with respect to the last two variables with suitable Printed in the U.S.A. ()1997 by North Atlantic Science Publishing Company 273

274 TADEUSZ JANKOWSKI V. LAKSHMIKANTHAM Lipschitz constants or Lipschitz functions [1-3, 5]. This paper applies the method of lower upper solutions for proving existence results [4]. Using this technique, we construct monotone sequences, giving sufficient conditions under which they are convergent. Moreover this method gives a problem solution in a closed set. Note that x(b) in condition (lb) may appear in a nonlinear way, so it is a reason that we consider the following problem in the place of (1): y (t) f(t, y(t), ix), t E J [0, b], where f C(J x n x R, n), G C(R x R, R). 2. Main Results A pair (v, c) e CI(j, R) x R is said to be a lower solution of (2) if: v (t) <_ f(t, v(t), c), t J, < an upper solution of (2) if the inequalities are reversed. Theorem 1" Assume that f C(J x R x R, R), G C(R x R, R), : 1 o yo, Zo e CI(j,R), ixo, 3 o R, such that (Yo, IXo), (Zo, 3 o) are lower upper solutions of problem (2) such that yo(t) < zo(t), t e J, ixo < /o; 2 o f is nondecreasing with respect to the last two variables; 3 o G is nondecreasing with respect to the first variable; 4 o G(y, ix)- G(y, ) <_ N(- ix) for yo(t) <_ y(t) <_ zo(t), t J, ixo <- ix <- fl <- /o with N >_ O. Then there exist monotone sequences {Yn, IXn}, {Zn, n} such that yn(t)-*y(t), zn(t)-*z(t), t E J; ixnix, ") n/ as noc; this convergence is uniformly monotonically on J. Moreover, (y, ix), (z,) are minimal maximal solutions of problem (2), respectively. Proof: From the above assumptions, it is known that: Yo(0) <- ko, Z o(t) >- f(t, zo(t), 7o), Zo(0) >- ko, 0 >_ G(zo, 7o), yo(t) < Zo(t), t J, ixo < 70. Let (yl,)l), (Zl, ) l) be the solutions of: Yi(t) f(t, Yo(t),ixo), Yl(0) 0 G(yo, ixo)- N(IXl z(t)- f(t, zo(t),7o), Zl(0 ]0 0 a(zo, o) N(71 70),

Monotone Iterations for Differential Equations with a Parameter 275 respectively. Put p- o- A1, so: 0 a(yo, Ao)- N(A 1 Ao) >- N(A1 Ao) Np, thus p _< 0 o -< A1- Now let p 1 71" In view of 3 o 4 o, we have: 0 G(Y0, )o) N(A1 Ao) G(Yo, Ao) G(zo 70) N()I "o) + N(71 70) K_ G(zo, )o) G(zo, 70) N(I o) q- N(71 70) -< N(7o- Ao)- N( I o) + N(71 70) Np. HenceA1_<71. Set p-71-70, sothat: 0 G(Zo, 70) N(71 70) _K N(71 70) Np, As a result, we have: AO A1 < 71 70" We shall show that thus 71 -< 70. yo(t) <_ yl(t) <_ zl(t.<_ zo(t), t E Z. (3) Let p(t)- yo(t)- yl(t), t E J, so: p (t) yo(t)- yl(t) <_ f(t, yo(t),ao)- f(t, yo(t),ao) O, p(0)- Yo(0)-Yl(0)<_ 0. This shows that p(t) <_ 0, t J. Therefore yo(t) <_ yl(t), t J. Put p(t) yl(t)- zl(t), t J. In view of 2 o, we have p (t) y(t)- Zl(t) f(t, yo(t),ao)- f(t, zo(t),7o) <_ f(t, zo(t), 70) f(t, zo(t), 70) O, p(0)-0, so p(t)<_o, tj, yl(t)<_zl(t), tej. Put p(t)-zl(t )-zo(t), t J. We obtain: p (t) Zl(t) Z o(t <_ f(t, zo(t), 70) f(t, zo(t), 7o) O, so p(t) 0, t J, hence zl(t <_ zo(t), t J. This shows that (3) is satisfied. Note that: yl(t)- f(t, Yo(t),)o) <_ f(t, Yl(t),l),Yl(O)- Moreover, in view of 3 o Zl(t) -f(t, zo(t),7o) >- f(t, zl(t), 71), Zl(0) ]0" 4 o, we have" 0 G(Yo, 0)- N(I A0) <- G(Yl, o)- N( 1.- o) G(yl, "o)- G(yl, A1)q-((Yl, "1)- N()I )o) -< N()I )o) q- G(Yl, 1) N()I )o) G(yl, 1), 0 G(z0, 70) N(71 70) >- G(Zl, 70) N(71 70) G(Zl, 70) G(Zl, 71) q- G(Zl, 71) N(71 70)

276 TADEUSZ JANKOWSKI V. LAKSHMIKANTHAM _ - -- -- -- -- o 1 - "- k- k 1 /k /k- 1 --" -- ") 1 0, yz(t) _< f(t, Yk(t), Ak) 0 G(Yk,,k) - for some k > 1. We shall prove that: _ k<--)ktl--<tk+l Yk( t) yk + l(t)-zk+l(t)-zk(t), re J, Y+I(t)- f(t, YkTl(t),)k+l), Yk+l(0)- 0 G(y k + k 1, + 1), z + l(t) >_ f(t,z k + l(t), 7k + 1), Zk + 1(0) 0 G(z k + l") k + 1) >-- N(/1 ") 0) + G(Zl, ") 1) N(71 7o) G(Zl, ") 1)" Consequently, (yl,l), (z1, ),1) are lower upper solutions of problem (2). Let us assume that Yo(t) < Yl(t) -< " "-< Yk-1(t) --< Yk(t) <-- zk(t) <-- Zk-1(t) --< ""--< zl(t) --< Zo(t), tej [ Yk(O)- ko, J z(t) >_ f(t, zk(t), 7k), Zk(O)- ko, o >_ a(z, (4) where Y + 1(t) f(t yk(t) ;k) Yk + 1(0) k0, 0 G(yk, "k)- N(k + 1 k) z + l(t) f(t, zk(t), 7k), Zk + 1(0) k0, 0 G(Zk, 7k) N(Tk + 1 7k)" Put p "k- "k + 1, so" 0 G(Yk,,k)- N(, k + 1 k) >-- N(Ak + 1 )k) Np, hence,k<_,k+l. Let P-- k+l--tk+l" 0 G(Yk, $k)- N(Ak + 1 )k) G(Yk, k) G(Zk, 7k) N(k + )k) -t- N(7 k + 1, <_ a(zk, "k) a(zk, 7) N(, + 1 k) q- N(Tk + 1 < N(Tk Ak) N(k + 1 )k) nu N(Tk + 1 3/k) Np. In view of 3 4,weseethat:

Monotone Iterations for Differential Equations with a Parameter 277 Hence we have A k + 1 -< 7k + 1" Now, let p 7k + 1 ") k" Then: 0 G(zk, 7k) N(Tk + 1 ")/k) <-- Np, so 7k + 1 < 7k, which shows that the first inequality of (4) is satisfied. As before, we set p(t) yk(t)- Yk -- + l(t), t E J. Then: p (t) yk(t)- Yk + 1(t) f(t Yk(t) Ak)- f(t Yk(t) Ak) O, p(0)-0, so Yk(t)<_Yk+l(t), tej. We observe that for p(t)--yk+l(t)- z k + l(t), t J, we have p (t) Yk + 1(t) z + l(t)- f(t, Yk(t),Ak)- f(t, zk(t), - <_ f(t, z(t), 7) f(t, z(t), 7) 0 p (t) z k + l(t) z(t) < f(t, zk(t), 7k) f(t, zk(t), 7k) 0, SO Z k h-1 (t) Zk(t) t C:. J. Therefore: Yk( t) <- yk + l(t)-<zk+l(t)-<zk(t), reg. It is simple to show that (Yk + 1,Ak + 1), (zk + 1,Tk + 1) are lower upper solutions of problem (2). which proves that Yk+l(t) <zk+l(t), tj. Put p(t)-zk+l(t)-zk(t), tj. Then we have: Hence, by induction, we have: "0 -</1 -< "-< )n -< "/n <-""-< 1 -< 0, Yo(t) <_ Yl(t) _< _< Yn(t) <_ Zn(t <_ <_ Zl(t _< Zo(t), t J for all n. Employing stard techniques [4], it can be shown that the sequences {Yn, An}, {Zn, 7n} converge uniformly monotonically to (y,a), (z, 7), respectively. Indeed, (y,a) (z, 7) are solutions of problem (2) in view of the continuity of f G, the definitions of the above sequences. We have to show that if (u, )is any solution of problem (2) such that: Yo(t) <_ u(t) <_ zo(t), t J, A 0 <_/ <_ 70, then: yo(t) <_ y(t) <_ u(t) <_ z(t) <_ zo(t), t J, A 0 _< A < < 7 < 70" To show this, we suppose that" yk(t) <_ u(t) < zk(t), t J, A k _</ _< 7k for some k. Put A k _ " + 1 Then, in view of 3 o 4 o, we have 0 G(Yk, "k)- N(/k -t- 1 "k) <-- a(u, )k)- N(/k -t- 1 "k) a(zt,/k)- a(u, fl)- N(. k h- 1 "k) <_ N(fl "k) N()k -t- "k) Np, so p <_ 0, hence Ak + 1 -< " Let p 7k + 1" Then we obtain" 0 G(tt, [3) a(zk, t) G(Zk, ) G(Zk, [k) at" N(fk h- 1 [k)

278 TADEUSZ JANKOWSKI V. LAKSHMIKANTHAM hence p <_ 0, so fl <_ 7k + 1" <_ N(Tk ) + N(Tk + 1 7k) Np, This shows that: "k+ <--/ <-- 7+1. As before, we set p(t) Yk + 1(t) u(t), t E J. In view of 2 we obtain" p(t) Yk h" 1 tt(t) f( Yk(t) /k) f(t, lt(t), < f(t, u(t), fl) f(t, u(t), fl) 0; hence p(t)<_o, re J, Yk+l(t)<-u(t), tj. Now let p(t)--u(t)-z k+ tj. We see that" p (t) u (t)- z k + (t) f(t,u(t),fl)- f(t, zk(t),7) < f(t,z(t), 7) f(t,z(t), 7a) 0, p(t) < 0, t J, so u(t) < zk + l(t), t J. This shows that: Yk + l(t) -< u(t) <_ z k + l(t), By induction, this proves that the inequalities: are satisfied for all n. tej. Yn(t) <_u(t) <_Zn(t), t J, n-<fl-<tn Taking the limit as nc, we conclude that: y(t) <_ u(t) <_ z(t), t J, _< _< 7. Therefore, (y,), (z, 7) are minimal maximal solutions of (2). The proof is cornplete. References [1] [4] Goma, A., The method of successive approximations in a two-point boundary value problem with a parameter, Ukr. Mat. Zh. 29 (1977), 800-807. (In Russian) Jankowski, T., Existence, uniqueness approximate solutions of problems with a parameter, Zesz. Nauk. Politech. Gdansk Mat. 16 (1993), 3-167. Kurpiel, N.S. Marusiak, A.G., A multipoint boundary value problem for differential equations with parameters, Ukr. Mat. Zh. 32 (1980), 223-226. (In Russian) Ladde, G.S., Lakshmikantham, V. Vatsala, A.S., Monotone Iterative Techniques for Nonlinear Differential Equations, Pitman, Boston 1985. Pomentale, T., A constructive theorem of existence uniqueness for the problem y = f(x,y,a), y(a),a,y(b)= fl, Z. Angew. Math. Mech. 56 (1976), 387-388.

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