Research article

Monotone Dynamical Systems with Polyhedral Order Cones and Dense Periodic Points

  • Received: 29 November 2016 Accepted: 30 November 2016 Published: 16 December 2016
  • Let XRn be a set whose interior is connected and dense in X, ordered by a closed convex cone KRn having nonempty interior. Let T:XX be an order-preserving homeomorphism. The following result is proved: Assume the set of periodic points of T is dense in X, and K is a polyhedron. Then T is periodic.

    Citation: Morris W. Hirsch. Monotone Dynamical Systems with Polyhedral Order Cones and Dense Periodic Points[J]. AIMS Mathematics, 2017, 2(1): 24-27. doi: 10.3934/Math.2017.1.24

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  • Let XRn be a set whose interior is connected and dense in X, ordered by a closed convex cone KRn having nonempty interior. Let T:XX be an order-preserving homeomorphism. The following result is proved: Assume the set of periodic points of T is dense in X, and K is a polyhedron. Then T is periodic.


    1. Introduction

    The following postulates and notation are used throughout:

    • KRn (Euclidean n-space) is a solid order cone: a closed convex cone that has nonempty interior IntK and contains no affine line.

    • Rn has the (partial) order determined by K:

    yxyxK,

    referred to as the K-order.

    • XRn is a nonempty set whose Int X is connected and dense in X.

    • T: XX is homeomorphism that is monotone for the K-order:

    xyTxTy.

    A point xX has period k provided k is a positive integer and Tkx=x. The set of such points is Pk=Pk(T), and the set of periodic points is P=P(T)=kPk. T is periodic if X=Pk, and pointwise periodic if X=P.

    Our main concern is the following speculation:

    Conjecture. If P is dense in X, then T is periodic.

    The assumptions on X show that T is periodic iff T|IntX is periodic. Therefore we assume henceforth:

    •  X is connected and open Rn.

    We prove the conjecture under the additional assumption that K is a polyhedron, the intersection of finitely many closed affine halfspaces of Rn:

    Theorem 1 (Main). Assume K is a polyhedron, T: XX is monotone for the K-order, and P is dense in X. Then T is periodic.

    For analytic maps there is an interesting contrapositive:

    Theorem 2. Assume K is a polyhedron and T: XX is monotone for the K-order. If T is analytic but not periodic, P is nowhere dense.

    Proof. As X is open and connected but not contained in any of the closed sets Pk, analyticity implies each Pk is nowhere dense. Since P=k=1Pk, a well known theorem of Baire [1] implies P is nowhere dense.

    The following result of D. Montgomery [4]*is crucial for the proof of the Main Theorem:

    *See also S. Kaul [3].

    Theorem 3 (Montgomery). Every pointwise periodic homeomorphism of a connected manifold is periodic.

    Notation

    i,j,k,l denote positive integers. Points of Rn are denoted by a,b,p,q,u,v,w,x,y,z.

    xy is a synonym for yx. If xy and xy we write x or yx.

    The relations xy and yx mean yxIntK.

    A set S is totally ordered if x,ySxy or xy.

    If xy, the order interval [x,y] is {z:xzy}=KxKy.

    The translation of K by xRn is Kx:={w+x,wK.}

    The image of a set or point ξ under a map H is denoted by Hξ or H(ξ). A set S is positively invariant under H if HSS, invariant if Hξ=ξ, and periodically invariant if Hkξ=ξ.


    2. Proof of the Main Theorem

    The following four topological consequences of the standing assumptions are valid even if K is not polyhedral.

    Proposition 4. Assune p,qPk are such that

    pq,p,qPk.[p,q]X.

    Then Tk([p,q]=[p,q].

    proof. It suffices to take k=1. Evidently TP=P, and T[p,q][p,q] because T is monotone, whence Int[p,q]P is positively invariant under T. The conclusion follows because Int[p,q]P is dense in [p,q] and T is continuous.

    Proposition 5. Assume a,bPk,ab, and [a,b]X. There is a compact arc JPk[a,b] that joins a to b, and is totally ordered by .

    proof. An application of Zorn's Lemma yields a maximal set J[a,b]P such that: J is totally ordered by , a=maxJ, b=minJ. Maximality implies J is compact and connected and a,bJ, so J is an arc (Wilder [7], Theorem I.11.23).

    Proposition 6. Let MX be a homeomorphically embedded topological manifold of dimension n1, with empty boundary.

    (i) P is dense in M.

    (ii) If M is periodically invariant, it has a neighborhood base B of periodically invariant open sets.

    proof. (i) M locally separates X, by Lefschetz duality [5] (or dimension theory [6]. Therefore we can choose a family V of nonempty open sets in X that the family of sets VM:={VM:VV) satisfies:

    •  VM is a neighborhood basis of M,

    •  each set VM separates V.

    By Proposition 5, for each VV there is a compact arc JVPV whose endpoints aV,bv lie in different components of V\M. Since JV is connected, it contains a point in VMP. This proves (i).

    (ii) With notation as above, let BV:=[aV,bV]\[aV,bV]. The desired neighborhood basis is B:={BV:VV}.

    From Propositions 4 and 6 we infer:

    Proposition 7. Suppose p,qP, pq and [p,q]X. Then P is dense in [p,q].

    Let T(m) stand for the statement of Theorem 1 for the case n=m. Then T(0) is trivial, and we use the following inductive hypothesis:

    Hypothesis (Induction). n1 and T(n1) holds.

    Let QRn be a compact n-dimensional polyhedron. Its boundary Q is the union of finitely many convex compact (n1)-cells, the faces of Q. Each face F is the intersection of [p,q] with a unique affine hyperplane En1. The corresponding open face F:=F\F is an open (n1)-cell in En1. Distinct open faces are disjoint, and their union is dense and open in Q.

    Proposition 8. Assume p,qPk, pq, [p,q]X. Then T|[p,q] is periodic.

    8224; This result is adapted from Hirsch & Smith [2],Theorems 5,11 & 5,15.

    proof. [p,q] is a compact, convex n-dimensional polyhedron, invariant under Tk (Proposition 4). By Proposition 6 applied to M:=[p,q], there is a neighborhood base B for [p,q] composed of periodically invariant open sets. Therefore if F[p,q] is an open face of [p,q], the family of sets

    BF:={WB:WF}

    is a neighborhood base for F, and each WBF is a periodically invariant open set in which P is dense.

    For every face F of [p,q] the Induction Hypothesis shows that FP. Therefore Montgomery's Theorem implies T|F is periodic, so T|F is periodic by continuity. Since [p,q] is the union of the finitely many faces, it follows that T|[p,q] is periodic.

    To complete the inductive proof of the Main Theorem, it suffices by Montgomery's theorem to prove that an arbitrary xX is periodic. As X is open in Rn and P is dense in X, there is an order interval [a,b]X such that

    axb,a,bPk.

    By Proposition 5, a and b are the endpoints of a compact arc JPk[a,b], totally ordered by . Define p,qJ:

    p:=sup{yJ:yx},q:=inf{yJ:yx}.

    If p=q=x then xPk. Otherwise pq, implying x[p,q], whence xP by Proposition 8


    Conflict of Interest

    The author declares no conflicts of interest in this paper.


    [1] R. Baire, Sur les fonctions de variables réelles, Ann. di Mat. 3 (1899), 1-123.
    [2] M. Hirsch and H. Smith, Monotone Dynamical Systems, Handbook of Differential Equations, volume 2, chapter 4. A. C?nada, P. Drabek & A. Fonda, editors. Elsevier North Holland, 2005.
    [3] S. Kaul, On pointwise periodic transformation groups, Proceedings of the American Mathematical Society 27 (1971), 391-394.
    [4] D. Montgomery, Pointwise periodic homeomorphisms, American Journal of Mathematics 59 (1937), 118-120.
    [5] E. Spanier, Algebraic Topology, McGraw Hill, 1966.
    [6] W. Hurewicz and H. Wallman, Dimension Theory, Princeton University Press, 1941.
    [7] R. Wilder, Topology of Manifolds, American Mathematical Society, 1949.
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