مرکز منطقه ای اطلاع رساني علوم و فناوري

Abstract :

Motivated by a question that naturally arises concerning certain nonlinear integral operators, we give an extension, to multilinear maps, of the Banach-Steinhaus principle of uniform boundedness for linear operators. Applications are considered, and of particular interest to us are operators $H$ that, for some positive integer $p$ , have the representation $(H_{x})(t)=\\int_{0}^{t} \\cdots \\int_{0}^{t} k (t, \\tau _{1}, \\cdots ,\\tau _{p})x(\\tau _{1})\\cdots x(\\tau _{p})d\\tau _{1} \\cdots d \\tau _{p}, t \\geq 0$ for an arbitrary bounded (Lebesgue measurable) complex-valued function $x$ on $[0, \\infty )$ , where the kernel $k$ has certain very reasonable integrability properties. We show, using the extension mentioned above, that such operators (which play an important role in the theory of representation of nonlinear systems) have the basic property that whenever they take the set of bounded functions into itself, there is a positive constant $c$ such that $\\parallel Hx \\parallel \\leq c\\parallel x \\parallel^{P}$ for all bounded $x$ , where $\\parallel \\cdot \\parallel$ denotes the usual sup norm; this had been proved earlier only for $p = 1$ . Related results for much more general cases are also given.