Densely defined operator

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".^{[clarification needed ]}

A closed operator that is used in practice is often densely defined.

A densely defined linear operator ${\displaystyle T}${\displaystyle T} from one topological vector space, ${\displaystyle X,}${\displaystyle X,} to another one, ${\displaystyle Y,}${\displaystyle Y,} is a linear operator that is defined on a dense linear subspace ${\displaystyle \operatorname {dom} (T)}${\displaystyle \operatorname {dom} (T)} of ${\displaystyle X}${\displaystyle X} and takes values in ${\displaystyle Y,}${\displaystyle Y,} written ${\displaystyle T:\operatorname {dom} (T)\subseteq X\to Y.}${\displaystyle T:\operatorname {dom} (T)\subseteq X\to Y.} Sometimes this is abbreviated as ${\displaystyle T:X\to Y}${\displaystyle T:X\to Y} when the context makes it clear that ${\displaystyle X}${\displaystyle X} might not be the set-theoretic domain of ${\displaystyle T.}${\displaystyle T.}

Consider the space ${\displaystyle C^{0}([0,1];\mathbb {R} )}${\displaystyle C^{0}([0,1];\mathbb {R} )} of all real-valued, continuous functions defined on the unit interval; let ${\displaystyle C^{1}([0,1];\mathbb {R} )}${\displaystyle C^{1}([0,1];\mathbb {R} )} denote the subspace consisting of all continuously differentiable functions. Equip ${\displaystyle C^{0}([0,1];\mathbb {R} )}${\displaystyle C^{0}([0,1];\mathbb {R} )} with the supremum norm ${\displaystyle \|,円\cdot ,円\|_{\infty }}${\displaystyle \|,円\cdot ,円\|_{\infty }}; this makes ${\displaystyle C^{0}([0,1];\mathbb {R} )}${\displaystyle C^{0}([0,1];\mathbb {R} )} into a real Banach space. The differentiation operator ${\displaystyle D}${\displaystyle D} given by $${\displaystyle (\mathrm {D} u)(x)=u'(x)}$${\displaystyle (\mathrm {D} u)(x)=u'(x)} is a densely defined operator from ${\displaystyle C^{0}([0,1];\mathbb {R} )}${\displaystyle C^{0}([0,1];\mathbb {R} )} to itself, defined on the dense subspace ${\displaystyle C^{1}([0,1];\mathbb {R} ).}${\displaystyle C^{1}([0,1];\mathbb {R} ).} The operator ${\displaystyle \mathrm {D} }${\displaystyle \mathrm {D} } is an example of an unbounded linear operator, since $${\displaystyle u_{n}(x)=e^{-nx}\quad {\text{ has }}\quad {\frac {\left\|\mathrm {D} u_{n}\right\|_{\infty }}{\left\|u_{n}\right\|_{\infty }}}=n.}$${\displaystyle u_{n}(x)=e^{-nx}\quad {\text{ has }}\quad {\frac {\left\|\mathrm {D} u_{n}\right\|_{\infty }}{\left\|u_{n}\right\|_{\infty }}}=n.} This unboundedness causes problems if one wishes to somehow continuously extend the differentiation operator ${\displaystyle D}${\displaystyle D} to the whole of ${\displaystyle C^{0}([0,1];\mathbb {R} ).}${\displaystyle C^{0}([0,1];\mathbb {R} ).}

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 ${\displaystyle i:H\to E}${\displaystyle i:H\to E} with adjoint ${\displaystyle j:=i^{*}:E^{*}\to H,}${\displaystyle j:=i^{*}:E^{*}\to H,} there is a natural continuous linear operator (in fact it is the inclusion, and is an isometry) from ${\displaystyle j\left(E^{*}\right)}${\displaystyle j\left(E^{*}\right)} to ${\displaystyle L^{2}(E,\gamma ;\mathbb {R} ),}${\displaystyle L^{2}(E,\gamma ;\mathbb {R} ),} under which ${\displaystyle j(f)\in j\left(E^{*}\right)\subseteq H}${\displaystyle j(f)\in j\left(E^{*}\right)\subseteq H} goes to the equivalence class ${\displaystyle [f]}${\displaystyle [f]} of ${\displaystyle f}${\displaystyle f} in ${\displaystyle L^{2}(E,\gamma ;\mathbb {R} ).}${\displaystyle L^{2}(E,\gamma ;\mathbb {R} ).} It can be shown that ${\displaystyle j\left(E^{*}\right)}${\displaystyle j\left(E^{*}\right)} is dense in ${\displaystyle H.}${\displaystyle H.} Since the above inclusion is continuous, there is a unique continuous linear extension ${\displaystyle I:H\to L^{2}(E,\gamma ;\mathbb {R} )}${\displaystyle I:H\to L^{2}(E,\gamma ;\mathbb {R} )} of the inclusion ${\displaystyle j\left(E^{*}\right)\to L^{2}(E,\gamma ;\mathbb {R} )}${\displaystyle j\left(E^{*}\right)\to L^{2}(E,\gamma ;\mathbb {R} )} to the whole of ${\displaystyle H.}${\displaystyle H.} This extension is the Paley–Wiener map.

• Blumberg 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 domain

• 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. ISBN 0-387-00444-0. MR 2028503.

• Functional analysis
• Hilbert spaces
• Linear operators
• Operator theory

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