\documentclass[reqno]{amsart} \usepackage{hyperref} \AtBeginDocument{{\noindent\small \emph{Electronic Journal of Differential Equations}, Vol. 2009(2009), No. 106, pp. 1--10.\newline ISSN: 1072-6691. URL: http://ejde.math.txstate.edu or http://ejde.math.unt.edu \newline ftp ejde.math.txstate.edu} \thanks{\copyright 2009 Texas State University - San Marcos.} \vspace{9mm}} \begin{document} \title[\hfilneg EJDE-2009/106\hfil Strong monotonicity] {Strong monotonicity for analytic ordinary differential equations} \author[S. Walcher, C. Zanders \hfil EJDE-2009/106\hfilneg] {Sebastian Walcher, Christian Zanders} % in alphabetical order \address{Sebastian Walcher \newline Lehrstuhl A f\"ur Mathematik, RWTH Aachen\\ 52056 Aachen, Germany} \email{walcher@matha.rwth-aachen.de} \address{Christian Zanders \newline Lehrstuhl A f\"ur Mathematik, RWTH Aachen\\ 52056 Aachen, Germany} \email{christian.zanders@matha.rwth-aachen.de} \thanks{Submitted August 21, 2009. Published September 1, 2009.} \subjclass[2000]{37C65, 37C25, 92C45, 34A12} \keywords{Monotone dynamical system; limit set; irreducible; \hfill\break\indent compartmental model} \begin{abstract} We present a necessary and sufficient criterion for the flow of an analytic ordinary differential equation to be strongly monotone; equivalently, strongly order-preserving. The criterion is given in terms of the reducibility set of the derivative of the right-hand side. Some applications to systems relevant in biology and ecology, including nonlinear compartmental systems, are discussed. \end{abstract} \maketitle \numberwithin{equation}{section} \newtheorem{theorem}{Theorem}[section] \newtheorem{lemma}[theorem]{Lemma} \newtheorem{corollary}[theorem]{Corollary} \section{Introduction}\label{intro} The qualitative theory of cooperative ordinary differential equations was initiated by Hirsch \cite{HirI}, \cite{HirII}, who proved a number of strong results on limit sets, in particular on convergence to stationary points. Hirsch, Smith and others extended the theory to monotone semiflows on ordered metric spaces; see the monograph by Smith \cite{SmMDS} and the article by Hirsch and Smith in \cite{HiSmHB} for an account and overview of the theory. The strong order-preserving (SOP) property for monotone semiflows is of particular importance in this context: As stated in Smith \cite[Ch.~1, Thm.~4.3]{SmMDS}, quasiconvergence is generic for SOP monotone semiflows that satisfy certain compactness properties for forward trajectories. The SOP property is closely related to (eventual) strong monotonicity. Limit sets of monotone dynamical systems may still be very complicated, even in the SOP scenario; see the recent paper by Enciso \cite{Enci} which extends a classical result by Smale \cite{Sma}. Moreover, the question of relaxing or replacing conditions for quasiconvergence or convergence is of continuing interest. Thus the investigation of limit sets for monotone dynamical systems continues to be a very active area of research. Some recent contributions are due to Jiang and Wang \cite{JiWa} on Kolmogorov systems (in particular in dimension three), to Hirsch and Smith \cite{HiSmEq} on the existence of asymptotically stable equilibria, and to Sontag and Wang \cite{SoWa} who showed that the limit set dichotomy is not always satisfied. Hirsch and Smith, in their survey \cite{HiSmHB}, improved and extended a number of results. The present note is concerned with a technical issue: How can strong monotonicity for cooperative ordinary differential equations $\dot x = f(x)$ be established? The basic result is due to Hirsch \cite{HirII}; see also Smith's monograph \cite[Ch.~4, Thm.~1.1]{SmMDS}: If the derivative $Df(x)$ is irreducible at every point then the local flow is strongly monotone and therefore SOP. It has been noted (see e.g. \cite[Ch.~4, Remark 1.1]{SmMDS}) that the condition can be relaxed. For the related problem of a non-autonomous cooperative linear system $\dot x = A(t)x$, Andersen and Sandqvist \cite{AnSan} proved that the following condition for strong monotonicity is necessary and sufficient: The matrix $A(t)$ is irreducible for all $t$ in an everywhere dense set. Hirsch and Smith gave several strong monotonicity criteria for non-autonomous and autonomous systems; see \cite[Lemma 3.7, Theorem 3.8, Corollary 3.11 and Theorem 3.13]{HiSmHB}. We will prove a necessary and sufficient strong monotonicity criterion for the autonomous analytic case, building on Smith \cite{SmMDS}, and Hirsch and Smith \cite{HiSmHB}: Informally speaking, the system is not strongly monotone if and only if its reducibility set (to be defined below) contains an invariant subset with certain geometric properties. Analyticity is required because the identity theorem will be used at some points. Moreover, analyticity allows a quite strong statement of the criterion, which therefore is useful in actual computations. We demonstrate this by a number of examples with relevance to biology and ecology. \section{Reducibility sets and strong monotonicity}\label{redu} Let us first introduce some notation and terminology. Given a positive integer $n$, let $N:=\{1,\ldots,n\}$. If $S$ is a nonempty and proper subset of $N$, we say that a matrix $C=(c_{ij})\in \mathbb{R}^{(n,n)}$ is {\em $S$-reducible} if $c_{ij}=0$ for all $i\in S$ and $j\in N\setminus S$. Hence the subspace $W_S:=\{x\in \mathbb{R}^n: x_i= 0 \text{ for all }i\in S\},$ is mapped to itself by an $S$-reducible matrix. Note that $C$ is reducible in the usual sense if it is $S$-reducible for some $\emptyset\subset S \subset N$. A reducible matrix $C$ may be $S_1$-reducible and $S_2$-reducible with different subsets $S_1$ and $S_2$ of $N$. In this case, one easily verifies that $C$ is also $S_1\cap S_2$-reducible if $S_1\cap S_2\neq \emptyset$. Now let $D\subseteq \mathbb{R}^p$ be open and nonempty, and $D\to \mathbb{R}^{(n,n)},\quad t\mapsto A(t)$ an analytic map. The {\em $S$-reducibility set} of $A$ is defined as $R_S =R_S(D):= \{ t\in D: A(t) \text{ is S-reducible}\},$ and the {\em reducibility set} $R=R(D)$ of $A$ is defined as the union of all $R_S$. One may extend this notion to $R_S(V)$ and $R(V)$ for subsets $V\subseteq D$. As usual, we denote by $P$ the closed positive orthant in $\mathbb{R}^n$, and write $z\leq w$ if $w-z\in P$, $z0$ for all $t>0$ in $D$. In case $i=j$ the second alternative holds. \noindent{\em (b)} Let $i, j$ be such that $x_{ij}=0$, and let $\widetilde S=\widetilde S(j):=\{ k\in N: x_{kj}=0\}.$ Then $a_{k\ell}=0$ for all $k\in \widetilde S$ and $\ell \in N\setminus \widetilde S$, hence $A(t)$ is $\widetilde S$-reducible for all $t\in D$. \end{lemma} \begin{proof} One has $$\label{vareq} \dot{x}_{ij}(t)=\sum_{\ell=1}^n a_{i\ell}(t) x_{\ell j}(t)$$ for all $t\in D$ and all $1\leq i,j \leq n$, and the $x_{ij}$ are analytic functions of $t$. If there is some $t_0\geq 0$ such that $x_{ij}(t_0)=0$, and all $x_{\ell j}(t_0)\geq 0$, then the equality in (\ref{vareq}) shows $\dot{x}_{ij}(t_0)\geq 0$. This is sufficient, by standard arguments on positive invariance, to ensure ${x}_{ij}(t)\geq 0$ for all $t\in D$, $t\geq 0$. (See \cite[Ch.~3, Remark 1.3]{SmMDS}, and \cite[Prop. 2.3.]{HiSmHB}) Now let $t_1 \in D$ with $t_1\geq 0$ such that $x_{ij}(t_1)>0$. Then (\ref{vareq}) shows $\dot{x}_{ij}(t)\geq a_{ii}(t)x_{ij}(t)$ and therefore $x_{ij}(t)>0 \text{ for all } t\geq t_1$ by properties of scalar differential inequalities. Thus, if $t_2>0$ and $x_{ij}(t_2)=0$ then $x_{ij}=0$ due to the identity theorem. As for part (b), we first note that $\widetilde S$ is nonempty by definition, and $\widetilde S\neq N$ due to $x_{ii}\neq 0$. Let $k\in \widetilde S$, thus $x_{kj}=0$. Then (\ref{vareq}) shows $0= \dot{x}_{kj}(t) = \sum_{\ell=1}^n a_{k\ell}(t)x_{\ell j}(t) = \sum_{\ell\in N\setminus \widetilde S} a_{k\ell}(t) x_{\ell j}(t).$ For all $t>0$ and $\ell \in N\setminus \widetilde S$ we have $x_{\ell j}(t)>0$ by part (a), thus $a_{k\ell}(t)=0$. \end{proof} \noindent{\em Remark.} From Andersen and Sandqvist \cite{AnSan} one sees that, in this scenario, the matrix $X(t)$ will also be $\widetilde S$-reducible. Essentially their argument uses the unique solution property of the differential equation. \smallskip Now consider an ordinary differential equation $$\label{deq} \dot x = f(x) \text{ on }U\subseteq \mathbb{R}^n,$$ with $U$ nonempty, open, connected and $P$-convex, and $f$ analytic. We denote the solution with initial value $y$ at $t=0$ by $\Phi(t, y)$, and call $\Phi$ the local flow of \eqref{deq}. Recall that $D_2\Phi(t, y)$ satisfies the variational equation $\frac{\partial}{\partial t}D_2\Phi(t, y) = Df\big(\Phi(t, y)\big)D_2\Phi(t, y)$ with initial value $E$. In this paper we will always assume that \eqref{deq} is cooperative on $U$, thus for $i,j\in N$ with $i\neq j$ and for all $x\in U$ the inequalities $\frac{\partial f_i}{\partial x_j}(x)\geq 0$ hold. We note that for every $y\in U$, Lemma \ref{basic} is applicable to the matrix $X(t)=D_2\Phi(t, y)$ with $A(t)= Df\big(\Phi(t, y)\big)$. Due to cooperativity, the local flow of \eqref{deq} is monotone. The local flow of the cooperative system \eqref{deq} is said to be strongly order-preserving (SOP) if for all $z,w\in U$ with $z0$ such that $\Phi(t_0,V_z)\leq \Phi(t_0,V_w)$. The following characterization is essentially known from \cite{SmMDS} or \cite{HiSmHB}. We include a proof of one implication for the reader's convenience. \begin{lemma}\label{smsop} For the cooperative analytic system \eqref{deq} the following are equivalent: \noindent{\em (i)} $\Phi$ is strongly monotone, thus for all $z, w\in U$ with $z0$. \noindent{\em (ii)} $\Phi$ is eventually strongly monotone, thus for all $z, w\in U$ with $z0$ such that $\Phi(t_0, z)\ll \Phi(t_0, w)$. \noindent{\em (iii)} $\Phi$ is SOP. \end{lemma} \begin{proof} (ii) $\Rightarrow$ (i)'': If $\Phi$ is not strongly monotone then there exist $z0$ such that $\left(\Phi(t_0, w)-\Phi(t_0, z)\right)_i=0 \quad \text{for some } i.$ Then monotonicity shows $\left(\Phi(t, w)-\Phi(t, z)\right)_i=0$ for $0\leq t \leq t_0$, thus for all $t>0$ by the identity theorem, and $\Phi$ is not eventually strongly monotone. \noindent (ii) $\Leftrightarrow$ (iii)'': See \cite[Ch.~1, Lemma 1.1]{SmMDS} and \cite[Prop. 1.2]{HiSmHB}. \end{proof} The starting point for any discussion of ordering properties is the following identity: $$\label{estm} \Phi(t,w) - \Phi(t,z) = \int_0^1 {D_2\Phi(t,z+s(w-z))} \cdot {(w-z)}\, {\rm d}s$$ One can use this to give a quite precise description of analytic monotone local flows that are not strongly monotone. \begin{theorem}\label{main} Let the cooperative analytic system \eqref{deq} be given on the $P$-convex, open and connected set $U$, and denote by $\Phi$ its local flow. Then the following are equivalent: \noindent{\em(a)} $\Phi$ is not strongly monotone. \noindent{\em(b)} There exist $z, w\in U$ with $z0$ and $0\leq s \leq 1$ then the same holds for $D_2\Phi\left(t, z+s(w-z)\right)$, as noted in the Remark following Lemma \ref{basic}. Now a straightforward application of (\ref{estm}) shows the assertion. For the reverse direction, assume that $\Phi$ is not strongly monotone. Then there exist $z, w\in U$ such that $w>z$ and $\Phi(t,w)-\Phi(t,z)\not\in\mathop{\rm int}P$ for all positive $t$ in some neighborhood of $0$. Let $T>0$ such that $\Phi(t,z+s(w-z))$ exists for $0\leq s\leq 1$ and $0\leq t0$. Hence $\Phi(t,w)-\Phi(t,z)\not\in\mathop{\rm int}P$ for some $t>0$ implies that $w-z\not\in\mathop{\rm int}P$. Therefore $S^*:=\left\{ i\in N: w_i-z_i=0\right\}$ is nonempty, and, by the same observation on diagonal entries of $B$, $\left(B(t, s) \cdot {(w-z)}\right)_j =0 \text{ for t>0 only if j\in S^*}$ whence $S:=\{ j\in N: \left(B(t, s) \cdot {(w-z)}\right)_j =0 \text{ for } t>0\}$ is a subset of $S^*$, and $S\neq \emptyset$ due to the hypothesis. Now $b_{jk}(t, s)=0$ for all $j\in S$, $k\in N\setminus S^*$, $0\leq s\leq 1$ and $t>0$, in view of $0 =\sum_\ell b_{j\ell}(w_\ell- z_\ell) =\sum_{k\in N\setminus S^*}b_{jk}(w_k- z_k).$ For $k\in N\setminus S^*$ define $\widetilde S(k):=\{j\in N: b_{jk}=0\}$. Lemma \ref{basic} and the proven part of the assertion show $\widetilde S(k)$-reducibility. From $S=\cap_{k\in N\setminus S^*}\widetilde S(k)$ we obtain $S$-reducibility. \end{proof} Note that in the scenario of Theorem \ref{main} certain matrix entries of $Df\left(\Phi(t, z+s(w-z))\right)$ vanish for all $(s, t)\in (0, 1)\times (0, T)$, and hence (by the identity theorem) for all $t$ where the solution is defined. Thus all $\Phi(t, z+s(w-z))$ lie in the $S$-reducibility set $R_S(U)$ for $x\mapsto Df(x)$. This means that $R_S(U)$ contains an invariant subset for \eqref{deq}, which in turn contains $z$ and $w$. We have shown: \begin{corollary}\label{maincor} Let the cooperative analytic system \eqref{deq} be given. Assume that for every nonempty proper subset $S$ of $N$ the $S$-reducibility set does not contain an invariant subset $Y$ such that $\{ z+ s(w-z): 0\leq s \leq 1\}\subseteq Y$ for some $z0$ then, as noted in Smith \cite{SmMDS}, this matrix is irreducible for all $x$, and thus the forward flow of (\ref{bcc}) is strongly monotone. Let us now replace the condition $g'>0$ by the more natural requirement that $g$ is strictly increasing, albeit at the expense of requiring analyticity. If $C$ is $S$-reducible for some set $S$ then $i\in S$ and $i>1$ imply $i-1\in S$ because of $c_{i,i-1}\neq 0$. This only leaves the possibilities $S=\{1,\ldots, k\}, \quad \text{some } kz and w-z\in W_S. Since the roots of g' are isolated, all elements of Y have the same n^{\rm th} component, say c, and the solution y=y(t,s) (see Corollary \ref{techcor}) satisfies y_n=c. This implies z_n=w_n. From (\ref{varvar}), we obtain \[ \frac{\partial}{\partial t} \frac{\partial}{\partial s}y(t,s)|_{t=0} =C\left(z+s(w-z)\right)\cdot(w-z).$ Since $y_n(t,s)$ is constant, the left hand side has entry $0$ at position $n$, as has $w-z$. The form of $C$ then implies that $w-z$ has entry $0$ at position $n-1$. Proceed by obvious induction to arrive at $z=w$; a contradiction. Thus no such set $Y$ exists, and we have strong monotonicity. \subsection*{Example 2: A modified Michaelis-Menten system} The three-dimensional system $$\label{sanmm} \begin{gathered} \dot x_1 = -x_1+(u+ax_1)x_2+b(1-x_1)h(x_3)\\ \dot x_2 = c(x_1-ax_1x_2-vx_2)\\ \dot x_3 = d(x_2-x_3) \end{gathered}$$ on the positive orthant of $\mathbb{R}^3$ describes a biochemical reaction through a membrane; see Sanchez \cite{San}. Here $a,b,c,d,u,v$ are positive constants, and $h$ is a decreasing function that sends $\mathbb{R}_+$ to itself. Following Sanchez \cite{San}, we focus interest on a certain positively invariant subset $U$ which is contained in $\{x\in \mathbb{R}^3: x_1>1, 00\}.$ On this set $U$ the derivative of the right-hand side is given by $C(x)=\begin{pmatrix} * & u+ ax_1 & b(1-x_1)h'(x_3)\\ c(1-ax_2) & * & 0 \\ 0 & d & * \end{pmatrix}$ and the forward flow is therefore monotone. Sanchez \cite{San} requires $h'<0$ to conclude irreducibility of all $C(x)$ and thus strong monotonicity of the forward flow on $U$, on the way to proving convergence to the set of equilibria for any initial value in $\mathbb{R}^3_+$. Again, we relax the condition on $h'$ at the expense of requiring analyticity; thus we assume $h' \leq 0$ but not identically zero, and $h$ strictly decreasing. The matrix $C(x)$ is reducible for $x\in U$ if and only if $h'(x_3)=0$, and in this case the matrix is $S$-reducible only for $S=\{1,2\}$. Assume that $Y\subseteq R_S(U)$ is invariant and connected. Then necessarily all elements of $Y$ have the same third entry, say $c$, thus all $z, w\in Y$ satisfy $w_3-z_3=0$. But then the condition $w-z\in W_S$ forces $z=w$, and Corollary \ref{maincor} shows strong monotonicity. \subsection*{Example 3: A cooperative Volterra-Lotka system with influx} Consider the $n$-dimensional system $$\label{vlin} \dot x_i= x_i\Big(\sum_j \beta_{ij}x_j + \gamma_i\Big)+\delta_i,\quad 1\leq i \leq n,$$ with real constants $\delta_i\geq 0$, $\gamma_i$ and $\beta_{ij}$ on (some open neighborhood of) the positive orthant $P$, with $\beta_{ij}\geq 0$ whenever $i\neq j$. In the case that all $\delta_i=0$ we have a Volterra-Lotka system for cooperating species. There is continued interest in Volterra-Lotka systems, both due to the (seeming) simplicity of their structure and to the challenges they pose to qualitative theory. We refer to the monograph \cite{HoSi} by Hofbauer and Sigmund for an introduction and an account of fundamental results. Note that Volterra-Lotka systems are special Kolmogorov systems. Abbreviating the right-hand side of (\ref{vlin}) by $f_i(x)$, $1\leq i \leq n$, one sees that $\frac{\partial f_i}{\partial x_j}= \beta_{ij}x_i,$ whenever $i\neq j$, hence the system is cooperative on the positive orthant. We now restrict attention to the special case of an irreducible matrix $\left(\beta_{ij}\right)$. In this case we have $R_S(P)= W_S\cap P.$ When all $\delta_i=0$ then all $R_S$ are invariant, as is well-known. Here one could say that the strong monotonicity criterion from Corollary \ref{maincor} fails completely (and so does strong monotonicity). But on the other hand, consider the system when all $\delta_i>0$ (influx of all species): Then no nonempty subset of the boundary of $P$ is invariant, and therefore Corollary \ref{maincor} shows that the forward flow is strongly monotone. This example illustrates the role of invariance in the criterion. \subsection*{Example 4: A nonlinear compartmental system} Consider the $n$-dimensional system $$\label{comp} \dot x_i =-\Big(\sum_{j\neq i}\rho_{ji}(x_i)+\gamma_i(x_i)\Big) +\sum_{j\neq i}\rho_{ij}(x_j)$$ on (some open neighborhood of) the positive orthant $P$. Thus we require the $\rho_{ij}$ and $\gamma_i$ to be defined and analytic on $(-\delta, \infty)$ for some $\delta>0$. Moreover we require that for all distinct $i$ and $j$ the $\rho_{ij}$ are nonnegative and increasing on $[0, \infty)$, with $\rho_{ij}(0)=0$. The differential equation thus describes a nonlinear compartmental system. Such systems are widely used in applications, e.g. in physiology and ecology; see the monographs by Anderson \cite{And}, and by Walter and Contreras \cite{WaCo}. Linear compartmental systems, which are very well-understood, satisfy $\rho_{ij}(x_j)=k_{ij}\cdot x_j$ with nonnegative constants $k_{ij}$ for $i\neq j$. But nonlinear systems are common in applications, and in fact most linear compartmental systems should be seen as limiting cases of nonlinear ones. If one views the underlying model as a collection of reservoirs separated by membranes then it is quite natural to assume monotonicity of the transport rate from one reservoir to the other: Higher concentration of the substance in the reservoir leads to a higher outflow rate. This property translates to monotonicity of the $\rho_{ij}$. Due to analyticity the $\rho_{ij}$ are either strictly monotone or identically zero. Abbreviating the right-hand side of (\ref{comp}) by $f(x)$, we have $\frac{\partial f_i}{\partial x_j}(x)=\rho_{ij}'(x_j)$ whenever $i\neq j$, and therefore the system is cooperative. We will show: If the forward flow of (\ref{comp}) is not strongly monotone then there is a nonempty proper subset $S^*$ of $N=\{1,\ldots,n\}$ such that $R_{S^*}(P)=P$, thus $Df(x)$ is $S^*$-reducible for all $x\in P$. In other words: Unless there is no flow at all from some subsystem with labels in $N\setminus S^*$ to the complementary subsystem with labels in $S^*$, the forward flow will be strongly monotone. As usual, the technical problem in the proof is due to possible isolated zeros of the $\rho_{ij}'$. Thus assume that there exists a connected invariant subset $Y$ of $P$, contained in some $R_S(P)$, and containing all $z+s(w-z)$ , where $zm. \end{gather*} Note that$m0\quad \text{for } j>m,\; t>0. \] Now (\ref{varvar}) shows directly that $S^*$-reducibility of $Df\left(y(t,s)\right)$, with $S^*:=\{1,\ldots, m\}$. Moreover, since $y_j$ is not constant for any $j>m$, we find that $\rho_{ij}'=0$ for all $i\in S^*$ and $j\notin S^*$. In other words, $Df(x)$ is $S^*$-reducible for all $x$. \begin{thebibliography}{00} \bibitem{AnSan} K.~M.~Andersen, A.~Sandqvist: \emph{A necessary and sufficient condition for a linear differential system to be strongly monotone.} Bull. London Math. 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