In mathematics – specifically, in operator theory – a densely defined operator or partially defined operator is a type of partially defined function. In a topological sense, it is a linear operator that is defined "almost everywhere". Densely defined operators often arise in functional analysis as operations that one would like to apply to a larger class of objects than those for which they a priori "make sense".
A densely defined linear operator from one topological vector space, to another one, is a linear operator that is defined on a dense linear subspace of and takes values in written Sometimes this is abbreviated as when the context makes it clear that might not be the set-theoretic domain of
is a densely defined operator from to itself, defined on the dense subspace The operator is an example of an unbounded linear operator, since
This unboundedness causes problems if one wishes to somehow continuously extend the differentiation operator to the whole of
The Paley–Wiener integral, on the other hand, is an example of a continuous extension of a densely defined operator. In any abstract Wiener space with adjoint there is a natural continuous linear operator (in fact it is the inclusion, and is an isometry) from to under which goes to the equivalence class of in It can be shown that is dense in Since the above inclusion is continuous, there is a unique continuous linear extension of the inclusion to the whole of This extension is the Paley–Wiener map.
See alsoedit
Blumberg theorem – Any real function on R admits a continuous restriction on a dense subset of R
Partial function – Function whose actual domain of definition may be smaller than its apparent domain
Referencesedit
Renardy, Michael; Rogers, Robert C. (2004). An introduction to partial differential equations. Texts in Applied Mathematics 13 (Second ed.). New York: Springer-Verlag. pp. xiv+434. ISBN0-387-00444-0. MR 2028503.
November 03, 2023
densely, defined, operator, mathematics, specifically, operator, theory, densely, defined, operator, partially, defined, operator, type, partially, defined, function, topological, sense, linear, operator, that, defined, almost, everywhere, often, arise, functi. In mathematics specifically in operator theory a densely defined operator or partially defined operator is a type of partially defined function In a topological sense it is a linear operator that is defined almost everywhere Densely defined operators often arise in functional analysis as operations that one would like to apply to a larger class of objects than those for which they a priori make sense Contents 1 Definition 2 Examples 3 See also 4 ReferencesDefinition editA densely defined linear operator T displaystyle T nbsp from one topological vector space X displaystyle X nbsp to another one Y displaystyle Y nbsp is a linear operator that is defined on a dense linear subspace dom T displaystyle operatorname dom T nbsp of X displaystyle X nbsp and takes values in Y displaystyle Y nbsp written T dom T X Y displaystyle T operatorname dom T subseteq X to Y nbsp Sometimes this is abbreviated as T X Y displaystyle T X to Y nbsp when the context makes it clear that X displaystyle X nbsp might not be the set theoretic domain of T displaystyle T nbsp Examples editConsider the space C 0 0 1 R displaystyle C 0 0 1 mathbb R nbsp of all real valued continuous functions defined on the unit interval let C 1 0 1 R displaystyle C 1 0 1 mathbb R nbsp denote the subspace consisting of all continuously differentiable functions Equip C 0 0 1 R displaystyle C 0 0 1 mathbb R nbsp with the supremum norm displaystyle cdot infty nbsp this makes C 0 0 1 R displaystyle C 0 0 1 mathbb R nbsp into a real Banach space The differentiation operator D displaystyle D nbsp given by D u x u x displaystyle mathrm D u x u x nbsp is a densely defined operator from C 0 0 1 R displaystyle C 0 0 1 mathbb R nbsp to itself defined on the dense subspace C 1 0 1 R displaystyle C 1 0 1 mathbb R nbsp The operator D displaystyle mathrm D nbsp is an example of an unbounded linear operator since u n x e n x has D u n u n n displaystyle u n x e nx quad text has quad frac left mathrm D u n right infty left u n right infty n nbsp This unboundedness causes problems if one wishes to somehow continuously extend the differentiation operator D displaystyle D nbsp to the whole of C 0 0 1 R displaystyle C 0 0 1 mathbb R nbsp The Paley Wiener integral on the other hand is an example of a continuous extension of a densely defined operator In any abstract Wiener space i H E displaystyle i H to E nbsp with adjoint j i E H displaystyle j i E to H nbsp there is a natural continuous linear operator in fact it is the inclusion and is an isometry from j E displaystyle j left E right nbsp to L 2 E g R displaystyle L 2 E gamma mathbb R nbsp under which j f j E H displaystyle j f in j left E right subseteq H nbsp goes to the equivalence class f displaystyle f nbsp of f displaystyle f nbsp in L 2 E g R displaystyle L 2 E gamma mathbb R nbsp It can be shown that j E displaystyle j left E right nbsp is dense in H displaystyle H nbsp Since the above inclusion is continuous there is a unique continuous linear extension I H L 2 E g R displaystyle I H to L 2 E gamma mathbb R nbsp of the inclusion j E L 2 E g R displaystyle j left E right to L 2 E gamma mathbb R nbsp to the whole of H displaystyle H nbsp This extension is the Paley Wiener map See also editBlumberg theorem Any real function on R admits a continuous restriction on a dense subset of R Closed graph theorem functional analysis Theorems connecting continuity to closure of graphs Linear extension linear algebra Mathematical function in linear algebraPages displaying short descriptions of redirect targets Partial function Function whose actual domain of definition may be smaller than its apparent domainReferences editRenardy Michael Rogers Robert C 2004 An introduction to partial differential equations Texts in Applied Mathematics 13 Second ed New York Springer Verlag pp xiv 434 ISBN 0 387 00444 0 MR 2028503 Retrieved from https en wikipedia org w index php title Densely defined operator amp oldid 1151314638, wikipedia, wiki, book, books, library,