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Minimal Periods for Ordinary Differential Equations in Strictly Convex Banach Spaces and Explicit Bounds for some l^p-Spaces

Let x(t) be a non-constant T-periodic solution to the ordinary differential equation x'= f(x) in a Banach space X where f is assumed to be Lipschitz continuous with constant L. Then there exists a constant c such that T L >= c, with c only depending on X. It is known that c >= 6 in any Banach space and that c = 2π in any Hilbert space, but whereas the bound of c = 2 pi is sharp in any Hilbert space, there exists only one known example of a Banach space such that c = 6 is optimal. In this paper, we show that the inequality is in fact strict in any strictly convex Banach space. Moreover, we improve the lower bound for l^p(R^n) and L^p(M, μ) for a range of p close to p = 2 by using a form of Wirtinger's inequality for functions in W^{1,p}([0, T ], L^p(M, μ)).