Nemytskii Operators on Some Function Spaces
For a nonempty set S, Banach spaces X and Y over a field K = R or C, and a nonempty open set \(U\,\subset\,X\), let G, F, and H be vector spaces of functions acting from S into X,from U into Y, and from S into Y, respectively. Usually, but not always, F, G, and H will be normed spaces. Given a function \((u,s)\longmapsto,\psi(u,s)\) from U × S into Y, and a function g : \(S\,\rightarrow\,U\), the Nemytskii operator \(N\psi\) is defined by \(N\psi(g)(s)\quad:=\quad,N\psi\,g)(s)\quad:=\quad\psi(g(s),s)\qquad s\,\epsilon\,S.\) (6.1) Other authors call such an operator a superposition operator, e.g. Appell and Zabrejko .We use the term Nemytskii operator (as many others have) partly to distinguish it from the two-function composition operator \((F,G)\mapsto FoG\) to be treated in Chapter 8. Recall also that for a linear operator A we write Ax := A(x). We will often apply this rule also when A is a Nemytskii operator.
KeywordsBanach Space Banach Algebra Minkowski Inequality Nonempty Open Subset Point Partition
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