Abstract. Let E be a Banach space ordered by a cone K, and let / : E -• E be locally Lipschitz continuous and quasimonotone increasing such that ^(f(y)-f(x))< -L^(y-x) (x < y) for a linear positive functional ^ and L > 0. We prove, under suitable conditions on K, f and "i, that / is a homeomorphism with decreasing and Lipschitz continuous inverse.
IntroductionLet (E, || • ||) be a real Banach space, ordered by a cone K. A cone K is a closed convex subset of E with AK C K (A > 0), and Kf)(-K) = {0}. As usual x < y :y -x € K. We will always assume that K is reproducing, that is K -K = E. Then, the set K* = { 0 (x > 0)} is a cone in the space of all continuous linear functionals E*, the dual cone.A functional \I/ 6 K* is called norrning if there are constants 0 < a < ¡3 such thatThe aim of this paper is to prove the following result:
THEOREM 1. Let f : E -» E be locally Lipschitz continuous, bounded on bounded subsets of E, and quasimonotone increasing. Let there exist a norrning functional6 K* and L > 0 such that
Then f : E -> E is a homeomorphism, and f 1 : E -> E is monotone decreasing and Lipschitz continuous. Moreover each initial value problemis uniquely solvable on [0, oo), and the solution satisfiesfor a constant M > 0.
Remarks:1. In particular Theorem 1 applies to linear mappings: Let A : E -• E be linear and continuous, let A* : E* -* E* be its adjoint, and let $ € K* be norming. If . A*\& < -L^/ for some L > 0, then A is an isomorphism. A related result for cones with nonempty interior can be found in [4].2. A finite dimensional version of Theorem 1 is due to the author [6]. In this result it is assumed that K has nonempty interior and that / is merely continuous. In the result above K may have empty interior.3. Functional conditions are a useful tool in the theory of quasimonotone increasing dynamical systems since in applications they lead to conditions which are often easy to deal with. For a survey on the subject we refer to Examples of ordered Banach spaces with reproducing cone and norming functionals are:REMARK: The cone in 4. is reproducing since it contains the reproducing cone Ko = {u : u(l) > > 0}, which is discussed in section 4. The following example shows that, in general, condition (1) in Theorem 1 does not lead to a bijective mapping in case that K is only assumed to be total, that is K -K = E. Consider E = co(N,M) endowed with the maximum norm and ordered by the coneThe cone K is total. To see this, recall that the finite sequences are dense in co(N, R).