Dynamics of a Rational Recursive Sequence

Transcription

1 International Journal of Difference Equations ISSN , Volume 4, Number 2, pp (2009) Dynamics of a Rational Recursive Sequence E M Elsayed Mansoura University Department of Mathematics, Faculty of Science Mansoura 35516, Egypt emelsayed@mansedueg Abstract In this paper, the dynamics of the difference equation ±1 ± x n 1 x n 3, n N 0, where the initial conditions are arbitrary nonzero real numbers, is studied Moreover, the solutions are obtained AMS Subject Classifications: 39A10 Keywords: Difference equations, recursive sequences, periodic solution 1 Introduction This paper studies the dynamics of the solutions of recursive sequences satisfying ±1 ± x n 1 x n 3, n N 0, (11) where the initial conditions are arbitrary nonzero real numbers Also, we get explicit forms of the solutions Recently there has been a great interest in studying the qualitative properties of rational difference equations The study of rational difference equations of order greater than one is quite challenging and rewarding because some prototypes for the development of the basic theory of the global behavior of nonlinear difference equations of order greater than one come from the results for rational difference equations However, there have not been any effective general methods to deal with the global behavior of Received July 19, 2008; Accepted October 7, 2009 Communicated by Martin Bohner

2 186 E M Elsayed rational difference equations of order greater than one so far Therefore, the study of rational difference equations of order greater than one is worth further consideration Aloqeili [1] obtained the solutions of the difference equation x n 1 a x n x n 1 Cinar [2 4] considered the solutions of the difference equations x n ax n x n 1, x n ax n x n 1, ax n bx n x n 1 Elabbasy et al [5] investigated the global stability, periodicity character and gave the solution of a special case of the recursive sequence ax n bx n cx n dx n 1 Elabbasy et al [7] studied the global stability, periodicity character, boundedness and obtained the solution of some special cases of the difference equation αx n k β + γ k x n i Elabbasy et al [8] investigated the global stability, periodicity character and gave the solution of some special cases of the difference equation dx n lx n k cx n s b + a Karatas et al [10] obtained the form of the solution of the difference equation 1 + x n 2 Simsek et al [17] found explicit forms of the solutions of the difference equation x n x n 1 For related work see also [13 22] For the systematical studies of rational and nonrational difference equations, one can refer to the papers [1 12] and references therein Here, we recall some notations and results which will be useful in our investigation Let I be some interval of real numbers and let f : I k+1 I be a continuously differentiable function Then for every set of initial conditions x k, x k+1,,x 0 I,

3 Dynamics of a Rational Recursive Sequence 187 the difference equation f(x n, x n 1,,x n k ), n N 0 (12) has a unique solution {x n } n= k [12] A point x I is called an equilibrium point of (12) if x = f(x, x,,x) That is, x n = x for n 0, is a solution of (12), or equivalently, x is a fixed point of f Definition 11 (Stability) (i) The equilibrium point x of (12) is called locally stable if for every ǫ > 0, there exists δ > 0 such that for all x k, x k+1,,x 1, x 0 I with we have x k x + x k+1 x + + x 0 x < δ, x n x < ǫ for all n k (ii) The equilibrium point x of (12) is called locally asymptotically stable if x is a locally stable solution of (12) and there exists γ > 0, such that for all with we have x k, x k+1,,x 1, x 0 I x k x + x k+1 x + + x 0 x < γ, lim x n = x n (iii) The equilibrium point x of (12) is called a global attractor if for all we have x k, x k+1,,x 1, x 0 I, lim n x n = x (iv) The equilibrium point x of (12) is called globally asymptotically stable if x is locally stable and x is also a global attractor of (12) (v) The equilibrium point x of (12) is called unstable if x is not locally stable The linearized equation of (12) about the equilibrium x is the linear difference equation k f(x, x,,x) y n+1 = y n i x n i

4 188 E M Elsayed Theorem 12 (see [11]) Assume that p, q R and k N 0 Then p + q < 1 is a sufficient condition for the asymptotic stability of the difference equation x n+1 + px n + qx n k = 0, n N 0 Remark 13 Theorem 12 can be easily extended to general linear equations of the form x n+k + p 1 x n+k p k x n = 0, n N 0, (13) where p 1, p 2,,p k R and k N Then (13) is asymptotically stable provided that k p i < 1 i=1 Definition 14 (Periodicity) A sequence {x n } n= k is said to be periodic with period p if x n+p = x n for all n k 2 The Difference Equation 1 + x n 1 x n 3 In this section we give a specific form of the solutions of the difference equation 1 + x n 1 x n 3, n N 0, (21) where the initial conditions are arbitrary nonzero positive real numbers Theorem 21 Let {x n } n= 5 be a solution of (21) Then for n N 0 x 6n 5 = x 6n 4 = x 6n 3 = n 1 n 1 f n 1, x 6n 2 = (1 + 3iace), x 6n 1 = (1 + (3i + 1)ace) e n 1, x 6n = (1 + (3i + 2)bdf) d n 1 n 1 c n 1 (1 + (3i + 1)ace), (1 + (3i + 2)ace) n 1 (1 + (3i + 2)bdf), (1 + (3i + 3)bdf) b n 1 n 1 (1 + (3i + 2)ace), (1 + (3i + 3)ace) a n 1 n 1 where x 5 = f, x 4 = e, x 3 = d, x 2 = c, x 1 = b, x 0 = a

5 Dynamics of a Rational Recursive Sequence 189 Proof For n = 0 the result holds Now suppose that n > 0 and that our assumption holds for n 1 That is, x 6n 11 = x 6n 10 = x 6n 9 = n 2 n 2 f n 2, x 6n 8 = (1 + 3iace), x 6n 7 = (1 + (3i + 1)ace) e n 2, x 6n 6 = (1 + (3i + 2)bdf) d n 2 n 2 c n 2 (1 + (3i + 1)ace), (1 + (3i + 2)ace) n 2 (1 + (3i + 2)bdf), (1 + (3i + 3)bdf) b n 2 n 2 (1 + (3i + 2)ace) (1 + (3i + 3)ace) a n 2 n 2 Now, it follows from (21) that x 6n 5 = = = = x 6n x 6n 7 x 6n 9 x 6n n 2 n 2 b n 2 n 2 (1 + (3i + 2)bdf) (1 + (3i + 3)bdf) n f n 2 f n 2 d n 2 n 2 (1 + (3i + 2)bdf) f n 2 n 2 ( 1 + b (1 + (3i + 3)bdf) bdf (1 + (3n 3)bdf) n 2 f n 2 df n 2 ) (1 + (3n 3)bdf) (1 + (3n 3)bdf)

6 190 E M Elsayed = = n 2 n 2 f n 1 {(1 + (3n 3)bdf) + bdf} f n 1 {1 + (3n 2)bdf} Hence, we have x 6n 5 = n 1 f n 1 Similarly, one can prove the other relations The proof is complete Theorem 22 Equation (21) has only the trivial equilibrium point which is always not locally asymptotically stable Proof For the equilibrium points of (21), we can write Then ie, x = x 1 + x 3 x + x 4 = x, x 4 = 0 Thus the equilibrium point of (21) is x = 0 Let f : (0, ) 3 (0, ) be the function defined by u f(u, v, w) = 1 + uvw Therefore it follows that f u (u, v, w) = We see that 1 (1 + uvw) 2, f v(u, v, w) = u2 w (1 + uvw) 2, f w(u, v, w) = f u (x, x, x) = 1, f v (x, x, xx) = 0, f w (x, x, x) = 0 The proof now follows by using Theorem 12 Theorem 23 Every positive solution x of (21) is bounded and lim n x n = 0 u 2 v (1 + uvw) 2

7 Dynamics of a Rational Recursive Sequence 191 Proof It follows from (21) that 1 + x n 1 x n 3 Then the subsequences {x 6n 5 } n=0, {x 6n 4} n=0, {x 6n 3} n=0, {x 6n 2} n=0, {x 6n 1} n=0, {x 6n } n=0 are decreasing and so are bounded from above by This completes the proof M = max{x 5, x 4, x 3, x 2, x 1, x 0 } Remark 24 Equation (21) has no prime period two solution To illustrate the results of this section, we now consider numerical examples which represent different types of solutions to (21) Example 25 Assume that x 5 = 9, x 4 = 8, x 3 = 3, x 2 = 15, x 1 = 7, x 0 = 4 See Figure 21 Figure 21: Solution of the Difference Equation 15 plot of x(n+1)= (x(n 5)/(1+x(n 1)*x(n 3)*x(n 5)) 1 + x n 1 x n 3 10 x(n) n Example 26 Assume that x 5 = 4, x 4 = 11, x 3 = 13, x 2 = 35, x 1 = 17, x 0 = 24 See Figure 22,

8 192 E M Elsayed Figure 22: Solution of the Difference Equation 4 plot of x(n+1)= (x(n 5)/(1+x(n 1)*x(n 3)*x(n 5)) 1 + x n 1 x n x(n) n 3 The Difference Equation 1 x n 1 x n 3 Here the specific form of the solutions of the difference equation 1 x n 1 x n 3, n N 0, (31) where the initial conditions are arbitrary nonzero real numbers, will be derived Theorem 31 Let {x n } n= 5 be a solution of (31) Then for n N 0 x 6n 5 = x 6n 4 = n 1 n 1 f n 1 (1 3ibdf), x 6n 2 = (1 (3i + 1)bdf) (1 3iace), x 6n 1 = (1 (3i + 1)ace) e n 1 c n 1 (1 (3i + 1)ace), (1 (3i + 2)ace) n 1 (1 (3i + 2)bdf), (1 (3i + 3)bdf) b n 1 n 1

9 Dynamics of a Rational Recursive Sequence 193 x 6n 3 = d n 1 (1 (3i + 1)bdf), x 6n = (1 (3i + 2)bdf) n 1 a n 1 (1 (3i + 2)ace), (1 (3i + 3)ace) n 1 where x 5 = f, x 4 = e, x 3 = d, x 2 = c, x 1 = b, x 0 = a, and jbdf 1, jace 1 for j N Proof The proof is similar to the proof of Theorem 21 and therefore it will be omitted Theorem 32 Equation (31) has a unique equilibrium point x = 0, which is not locally asymptotically stable Remark 33 Equation (31) has no prime period two solution Example 34 Figure 31 shows the solution when x 5 = 9, x 4 = 8, x 3 = 3, x 2 = 5, x 1 = 7, x 0 = 4 Figure 31: Solution of the Difference Equation 10 plot of x(n+1)= (x(n 5)/(1 x(n 1)*x(n 3)*x(n 5)) 1 x n 1 x n x(n) n Example 35 Figure 32 shows the solution when x 5 = 11, x 4 = 013, x 3 = 13, x 2 = 05, x 1 = 17, x 0 = 21

10 194 E M Elsayed Figure 32: Solution of the Difference Equation 5 plot of x(n+1)= (x(n 5)/(1 x(n 1)*x(n 3)*x(n 5)) 1 x n 1 x n x(n) n 4 The Difference Equation 1 + x n 1 x n 3 In this section, we investigate the solutions of the difference equation 1 + x n 1 x n 3, n N 0, (41) where the initial conditions are arbitrary nonzero real numbers with x 5 x 3 x 1 1 and x 4 x 2 x 0 1 Theorem 41 Every solution {x n } n= 5 of the form of (41) is periodic with period twelve and is { f, e, d, c, b, a, f 1 + bdf, e, d( 1 + bdf), 1 + ace c( 1 + ace), b 1 + bdf, } a, f, e, 1 + ace

11 Dynamics of a Rational Recursive Sequence 195 Proof From (41), we see that x 1 = x 5 = f 1 + bdf, x e 2 = 1 + ace, x 3 = d( 1 + bdf), x 4 = c( 1 + ace), b 1 + bdf, x a 6 = 1 + ace, x 7 = f = x 5, x 8 = e = x 4 Hence, the proof is completed Theorem 42 Equation (41) has two equilibrium points which are 0, 3 2, and these equilibrium points are not locally asymptotically stable Proof The proof is the same as the proof of Theorem 22 and hence is omitted Theorem 43 Equation (41) has a periodic solution of period six iff ace = bdf = 2, and then takes the form {f, e, d, c, b, a, f, e, d, c, b, a, } Proof The proof is obtained from Theorem 41 Remark 44 Equation (41) has no prime period two solution Example 45 Figure 41 shows the solution when x 5 = 11, x 4 = 08, x 3 = 13, x 2 = 05, x 1 = 17, x 0 = 21 Example 46 Figure 42 shows the solution when x 5 = 01, x 4 = 03, x 3 = 2, x 2 = 5, x 1 = 10, x 0 = 4/3 5 The Difference Equation 1 x n 1 x n 3 In this section, we study the solutions of the difference equation 1 x n 1 x n 3, n N 0, (51) where the initial conditions are arbitrary nonzero real numbers with x 5 x 3 x 1 1 and x 4 x 2 x 0 1 Theorem 51 Every solution {x n } n= 5 of (51) is periodic with period twelve and is of the form { f f, e, d, c, b, a, 1 + bdf, e, d(1 + bdf), 1 + ace c(1 + ace), } b 1 + bdf, a, f, e, 1 + ace

12 196 E M Elsayed Figure 41: Solution of the Difference Equation 4 plot of x(n+1)= (x(n 5)/( 1+x(n 1)*x(n 3)*x(n 5)) 1 + x n 1 x n x(n) n Proof The proof is similar to the proof of Theorem 41 and therefore is omitted Theorem 52 Equation (51) has one equilibrium point which is 0, and this equilibrium point is not locally asymptotically stable Theorem 53 Equation (51) has a periodic solution of period six iff ace = bdf = 2, and then takes the form {f, e, d, c, b, a, f, e, d, c, b, a, } Proof The proof is obtained from Theorem 51 Remark 54 Equation (51) has no prime period two solution Example 55 We consider x 5 = 31, x 4 = 13, x 3 = 02, x 2 = 9, x 1 = 7, x 0 = 04 See Figure 51 Example 56 See Figure 52 for the initial conditions x 5 = 02, x 4 = 07, x 3 = 3, x 2 = 2, x 1 = 10/3, x 0 = 10/7 References [1] Marwan Aloqeili Dynamics of a rational difference equation Appl Math Comput, 176(2): , 2006

13 Dynamics of a Rational Recursive Sequence 197 Figure 42: Solution of the Difference Equation 10 plot of x(n+1)= (x(n 5)/( 1+x(n 1)*x(n 3)*x(n 5)) 1 + x n 1 x n x(n) n

14 198 E M Elsayed Figure 51: Solution of the Difference Equation 60 plot of x(n+1)= (x(n 5)/( 1 x(n 1)*x(n 3)*x(n 5)) 1 x n 1 x n x(n) n [2] Cengiz Çinar On the positive solutions of the difference equation ax n bx n x n 1 Appl Math Comput, 156(2): , 2004 [3] Cengiz Çinar On the positive solutions of the difference equation x n ax n x n 1 Appl Math Comput, 158(3): , 2004 [4] Cengiz Çinar On the solutions of the difference equation Appl Math Comput, 158(3): , 2004 x n ax n x n 1 [5] E M Elabbasy, H El-Metwally, and E M Elsayed On the difference equation ax n bx n /(cx n dx n 1 ) Adv Difference Equ, pages Art ID 82579, 10, 2006 [6] E M Elabbasy, H El-Metwally, and E M Elsayed Global attractivity and periodic character of a fractional difference equation of order three Yokohama Math J, 53(2):89 100, 2007 [7] E M Elabbasy, H El-Metwally, and E M Elsayed On the difference equation αx n k β + γ k x J Concr Appl Math, 5(2): , 2007 n i

15 Dynamics of a Rational Recursive Sequence 199 Figure 52: Solution of the Difference Equation 35 plot of x(n+1)= (x(n 5)/( 1 x(n 1)*x(n 3)*x(n 5)) 1 x n 1 x n x(n) n [8] E M Elabbasy, H El-Metwally, and E M Elsayed Qualitative behavior of higher order difference equation Soochow J Math, 33(4): , 2007 [9] E A Grove, G Ladas, L C McGrath, and H A El-Metwally On the difference y n (2k+1) + p equation y n+1 = In Proceedings of the Sixth International y n (2k+1) + qy n 2l Conference on Difference Equations, pages , Boca Raton, FL, 2004 CRC [10] Ramazan Karatas, Cengiz Cinar, and Dağıstan Simsek On positive solutions of the difference equation Int J Contemp Math Sci, 1(9-1 + x n 2 12): , 2006 [11] V L Kocić and G Ladas Global behavior of nonlinear difference equations of higher order with applications, volume 256 of Mathematics and its Applications Kluwer Academic Publishers Group, Dordrecht, 1993 [12] M R S Kulenović and G Ladas Dynamics of second order rational difference equations Chapman & Hall/CRC, Boca Raton, FL, 2002 With open problems and conjectures

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