Abstract
A multi-valued iterative functional equation of order n is considered. A result on the existence and uniqueness of K-convex solutions in some class of multifunctions is presented.
MSC:39B12, 37E05, 54C60.
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1 Introduction
As indicated in the books [1, 2] and the surveys [3, 4], the polynomial-like iterative equation
where S is a subset of a linear space over ℝ, is a given function, s () are real constants, is the unknown function, and is the i th iterate of f, i.e., and for all , is one of the important forms of a functional equation since the problem of iterative roots and the problem of invariant curves can be reduced to the kind of equations. Many works have been contributed to studying single-valued solutions for Eq. (1.1); for example, in [5–11] for the case of linear F, [12, 13] for , [14] for general n, [15, 16] for smoothness, [17] for analyticity, [18–20] for convexity, [21–23] in high-dimensional spaces. However, a multifunction (called multi-valued function or set-valued map sometimes) is an important class of mappings often used in control theory [24], stochastics [25], artificial intelligence [26], and economics [27]. Hence, it gets more interesting to study multi-valued solutions for Eq. (1.1), i.e., the equation
where is an integer, s () are real constants, G is a given multifunction, and F is an unknown multifunction. Here the i th iterate of the multifunction F is defined recursively as
and for all . In 2004, Nikodem and Zhang [28] discussed Eq. (1.2) for with an increasing upper semi-continuous (USC) multifunction G on and proved the existence and uniqueness of USC solutions under the assumption that G has fixed points a and b and , are both constants such that and . As pointed out in [29], the generalization to USC multifunctions for Eq. (1.1) is rather difficult even if . Hence, discussing Eq. (1.2) for evokes great interest, but the greatest difficulty is that the multifunction has no Lipschitz condition. In 2011, this difficulty was overcome by introducing the class of unblended multifunctions, the existence of USC multi-valued solutions for a modified form of the equation
was proved in [29]. K-convex multifunctions, which are generalization of vector-valued convex functions, have wide applications in optimization (cf. [30]) and play an important role in various questions of convex analysis (cf. [31]). However, up to now, there are no results on convexity of multi-valued solutions for the iterative equation (1.2). In this note, we study the convexity of multi-valued solutions for Eq. (1.2). We prove the existence and uniqueness of K-convex solutions in some class of multifunctions for Eq. (1.3).
2 K-convex multifunctions
As in [30], let X and Y be linear spaces and be a convex cone, i.e., and for all . Let be a convex set. A multifunction is said to be K-convex on Ω if
A convex multifunction [32] may be stated as θ-convex and the convexity of a real-valued function may be stated as -convex, and concavity as -convex, where and . Let be the set of all multifunctions , where denotes the family of all nonempty closed subintervals of I.
Considering -convex multifunctions and -convex multifunctions, the following lemmas are obvious.
Lemma 2.1 Let . Then the multifunction is -convex on I if and only if
Lemma 2.2 Let . Then the multifunction is -convex on I if and only if
3 Some lemmas
In order to prove our main results, we give the following useful property (cf. [33, 34]).
Lemma 3.1 For and for an arbitrary real λ, the following properties hold:
-
(a)
,
-
(b)
,
-
(c)
,
where
As defined in [[32], Definition 3.5.1], a multifunction is increasing (resp. strictly increasing) if (resp. ) for all with . A multifunction is upper semi-continuous (USC) at a point if for every open set with , there exists a neighborhood of such that for every . F is USC on I if it is USC at every point in I. For convenience, let
and
Remark 3.1 If (resp. ), , then F must be single-valued on (resp. ).
Lemma 3.2 (resp. ) for (resp. ).
Proof By Lemma 2.2 in [29], we only need to prove that is -convex on I (resp. -convex on I). We first prove that is -convex on I for . By Lemma 2.1, the fact that is -convex on I implies that
Hence, for all ,
holds. Note that is strictly increasing. Consequently,
So
By
we have
because is -convex. Hence, by (3.1)
is proved.
Next, we prove is -convex on I for . By Lemma 2.2, the fact that is -convex on I implies that
Hence, for all ,
holds. Note that is strictly increasing. Consequently,
So
By
it follows that
because is -convex. Hence, by (3.2)
This completes the proof of . □
Define
where and .
Remark 3.2 The condition for ( for ) guarantees that the iterations , , are also multifunctions.
Lemma 3.3 and are complete metric spaces equipped with the distance
Proof By Lemma 3.1 in [29], we only need to prove that if such that in , i.e.,
then is -convex on I, where or . We first prove the case of . By (3.3), we have , . Hence,
Note that by Lemma 3.1,
and
Hence,
By (3.4) and (3.5), we have for every , there exists such that
and
, . Consequently,
because is -convex on I. Hence,
which shows that is -convex on I.
Next we prove the case of . By (3.6) and (3.7), we have for every ,
because is -convex on I. Hence,
which shows that is -convex on I. The proof is completed. □
Define
is a closed subset of . is a closed subset of .
By Lemma 3.2, one can prove the following result.
Lemma 3.4 (resp. ) if (resp. USIC−∗ ).
Lemma 3.5 If (resp. ), then
The proof of Lemma 3.5 is similar to that of Lemma 3.3 in [29]. We omit it here.
4 Convex solutions
Theorem 4.1 Suppose that , () and and with . Then for arbitrary constants satisfying
Eq. (1.3) has a unique solution if
Proof Define the mapping by
By Lemma 3.2, , are strictly increasing -convex on I because is strictly increasing -convex. Since is -convex on I and , we have
Hence, is -convex on I. Obviously, is strictly increasing and for . Similar to the proof of Theorem 4.1 in [29], by Lemma 3.4 and condition (4.1), . Thus, we have proved that is a self-mapping on . By Lemma 3.5 and condition (4.2), L is a contraction map. By Lemma 3.3, is a complete metric space. Using Banach’s fixed point principle, L has a unique fixed point F in , i.e.,
This completes the proof. □
We note the fact that if the sets A, B, C satisfy . Hence, every solution F of Eq. (1.3) satisfies
We have the following result.
Corollary 4.1 Under the same conditions as in Theorem 4.1, there exists a multifunction such that (4.4) holds.
For multifunctions in the other class , we have a similar result to Theorem 4.1. It can be proved similarly.
Theorem 4.2 Suppose that , () and and with . Then for arbitrary constants satisfying (4.1), Eq. (1.3) has a unique solution if condition (4.2) holds.
Corollary 4.2 Under the same conditions as in Theorem 4.2, there exists a multifunction such that (4.4) holds.
Remark 4.1 Although the assumption (or ) implies that F is single-valued on (or ), but Eq. (1.3) cannot be considered on the interval (or ) as a single-valued case and the point b (or a) as a multi-valued case, respectively, because there is no meaning at the point b (or a).
Remark 4.2 By Remark 3.1, there is no strictly increasing -convex multifunction in . The same applies to the case of . Consequently, Eq. (1.3) has no solution in (resp. ).
Remark 4.3 By Theorem 4.1 and Theorem 4.2, we actually only prove the existence and uniqueness of K-convex ( and , i.e., K is not a nontrivial convex cone) multi-valued solutions for Eq. (1.3). In fact, there is no convex multi-valued (i.e., -convex multi-valued) solutions for Eq. (1.3) in the multifunction class . Since is a convex multi-valued function on I if and only if
Hence, if Eq. (1.3) has a convex multi-valued solution F in , then F must be strictly increasing on I, which is contradictory to (4.5).
Remark 4.4 We point out that we actually only have proved a special class of K-convex solutions, i.e., strictly increasing K-convex solutions of Eq. (1.3). It is very difficult to discuss K-convex solutions of Eq. (1.3) which are not strictly increasing because the method in [29] cannot be used. Discussing non-strictly-increasing K-convex solutions of Eq. (1.3) will be the subject of our next work.
5 Examples
We give an example to illustrate the applications of Theorem 4.1. Consider the equation
where , , , and
Clearly, , where
Let and . It is easy to check that both (4.1) and (4.2) hold. Thus, by Theorem 4.1, Eq. (5.1) has a unique solution .
Remark 5.1 Example (5.1) cannot be solved by known single-valued results.
References
Kuczma M, Choczewski B, Ger R Encyclopedia Math. Appl. 32. In Iterative Functional Equations. Cambridge University Press, Cambridge; 1990.
Targonski G: Topics in Iteration Theory. Vandenhoeck & Ruprecht, Göttingen; 1981.
Baron K, Jarczyk W: Recent results on functional equations in a single variable, perspectives and open problems. Aequ. Math. 2001, 61: 1–48. 10.1007/s000100050159
Zhang J, Yang L, Zhang W: Some advances on functional equations. Adv. Math. (China) 1995, 24: 385–405.
Dhombres JG: Itération linéaire d’ordre deux. Publ. Math. (Debr.) 1977, 24: 177–187.
Jarczyk W: On an equation of linear iteration. Aequ. Math. 1996, 51: 303–310. 10.1007/BF01833285
Matkowski J, Zhang W: On linear dependence of iterates. J. Appl. Anal. 2000, 6: 149–157.
Mukherjea A, Ratti JS: On a functional equation involving iterates of a bijection on the unit interval. Nonlinear Anal. 1983, 7: 899–908. 10.1016/0362-546X(83)90065-2
Mukherjea A, Ratti JS: On a functional equation involving iterates of a bijection on the unit interval II. Nonlinear Anal. 1998, 31: 459–464. 10.1016/S0362-546X(96)00322-7
Tabor J, Tabor J: On a linear iterative equation. Results Math. 1995, 27: 412–421.
Yang D, Zhang W: Characteristic solutions of polynomial-like iterative equations. Aequ. Math. 2004, 67: 80–105. 10.1007/s00010-003-2708-4
Malenica M:On the solutions of the functional equation . Mat. Vesn. 1982, 6: 301–305.
Zhao L:A theorem concerning the existence and uniqueness of solutions of the functional equation . J. Univ. Sci. Tech. 1983, 32: 21–27. in Chinese
Zhang W:Discussion on the iterated equation . Chin. Sci. Bull. 1987, 32: 1444–1451.
Mai J, Liu X:Existence, uniqueness and stability of solutions of iterative functional equations. Sci. China Ser. A 2000, 43: 897–913. 10.1007/BF02879796
Zhang W:Discussion on the differentiable solutions of the iterated equation . Nonlinear Anal. 1990, 15: 387–398. 10.1016/0362-546X(90)90147-9
Si J:Existence of locally analytic solutions of the iterated equation . Acta Math. Sin. 1994, 37: 590–600. in Chinese
Trif T: Convex solutions to polynomial-like iterative equations on open intervals. Aequ. Math. 2010, 79: 315–325. 10.1007/s00010-010-0020-7
Xu B, Zhang W: Decreasing solutions and convex solutions of the polynomial-like iterative equation. J. Math. Anal. Appl. 2007, 329: 483–497. 10.1016/j.jmaa.2006.06.087
Zhang W, Nikodem K, Xu B: Convex solutions of polynomial-like iterative equations. J. Math. Anal. Appl. 2006, 315: 29–40. 10.1016/j.jmaa.2005.10.006
Kulczycki M, Tabor J: Iterative functional equations in the class of Lipschitz functions. Aequ. Math. 2002, 64: 24–33. 10.1007/s00010-002-8028-2
Tabor J, Żoldak M: Iterative equations in Banach spaces. J. Math. Anal. Appl. 2004, 299: 651–662. 10.1016/j.jmaa.2004.06.011
Zhang W: Solutions of equivariance for a polynomial-like iterative equation. Proc. R. Soc. Edinb. A 2000, 130: 1153–1163. 10.1017/S0308210500000615
Johansson KH, Rantzer A, Aström KJ: Fast switches in relay feedback systems. Automatica 1999, 35: 539–552. 10.1016/S0005-1098(98)00160-5
Choi C, Nam D: Interpolation for partly hidden diffusion processes. Stoch. Process. Appl. 2004, 113: 199–216. 10.1016/j.spa.2004.03.014
Lawry J: A framework for linguistic modelling. Artif. Intell. 2004, 155: 1–39. 10.1016/j.artint.2003.10.001
Starr RM: General Equilibrium Theory. Cambridge University Press, Cambridge; 1997.
Nikodem K, Zhang W: On a multivalued iterative equation. Publ. Math. 2004, 64: 427–435.
Xu B, Nikodem K, Zhang W: On a multivalued iterative equation of order n . J. Convex Anal. 2011, 18: 673–686.
Borwein J: Multivalued convexity and optimization: a unified approach to inequality and equality constraints. Math. Program. 1977, 13: 183–199. 10.1007/BF01584336
Kuroiwa D, Tanaka T, Ha TXD: On cone convexity of set-valued maps. Nonlinear Anal., Theory Methods Appl. 1997, 30: 1487–1496. 10.1016/S0362-546X(97)00213-7
Aubin JP, Frankowska H: Set-Valued Analysis. Birkhäuser, Boston; 1990.
Radström H: An embedding theorem for space of convex sets. Proc. Am. Math. Soc. 1952, 3: 165–169.
Smajdor W: Local set-valued solutions of the Jensen and Pexider functional equations. Publ. Math. 1993, 43: 255–263.
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The author is most grateful to the Editor for the careful reading of the manuscript and anonymous referees for valuable suggestions that helped in significantly improving an earlier version of this paper. This work was supported by Key Project of Sichuan Provincial Department of Education (12ZA086) (China).
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Gong, X. Convex solutions of the multi-valued iterative equation of order n. J Inequal Appl 2012, 258 (2012). https://doi.org/10.1186/1029-242X-2012-258
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DOI: https://doi.org/10.1186/1029-242X-2012-258