Rigged Hilbert space

{{multiple issues| {{Technical|date=June 2020}} {{No footnotes|date=July 2023}} }} {{Short description|Construction for adding objects to a Hilbert space}} In mathematics and physics, a '''rigged Hilbert space''' ('''Gelfand triple''', '''nested Hilbert space''', '''equipped Hilbert space''') is a construction which can enlarge a Hilbert space to a bigger space containing additional objects which are not in the Hilbert space but which one would like to think of alongside the Hilbert space. For example, in the quantum mechanical description of a non-relativistic particle using the Hilbert space of square-integrable functions on the real line, eigenstates of the position and momentum operators are not in the Hilbert space, but are in a suitably defined rigged Hilbert space. Informally, the term "rigged" means that the Hilbert space has been equipped to do more than it otherwise could, in analogy with rigging a boat.<ref>{{Cite book |last=Ballentine |first=Leslie E. |title=Quantum mechanics: a modern development |date=2010 |publisher=World Scientific |isbn=978-981-02-4105-6 |edition=repr |location=Singapore}}</ref>

This construction is designed to link the distribution and square-integrable aspects of functional analysis. Such spaces were introduced to study spectral theory. They bring together the 'bound state' (eigenvector) and 'continuous spectrum', in one place.

Using this notion, a version of the spectral theorem for unbounded operators on Hilbert space can be formulated.<ref>{{eom|id=Rigged Hilbert space|first=R. A.|last=Minlos|authorlink=Robert Minlos}}</ref> "Rigged Hilbert spaces are well known as the structure which provides a proper mathematical meaning to the Dirac formulation of quantum mechanics."<ref>{{cite book |last1=Krasnoholovets |first1=Volodymyr |last2=Columbus |first2=Frank H. |title=New Research in Quantum Physics |date=2004 |publisher=Nova Science Publishers |isbn=978-1-59454-001-1 |page=79 |language=en}}</ref>

==Motivation==

A function such as <math display="block"> x \mapsto e^{ix} , </math> is an eigenfunction of the differential operator <math display="block">-i\frac{d}{dx}</math> on the real line {{math|'''R'''}}, but isn't square-integrable for the usual (Lebesgue) measure on {{math|'''R'''}}. To properly consider this function as an eigenfunction requires some way of stepping outside the strict confines of the Hilbert space theory. This was supplied by the apparatus of distributions, and a ''generalized eigenfunction'' theory was developed in the years after 1950.{{sfn|Gel'fand|Vilenkin|1964|pp=103-105}}

==Definition==

A '''rigged Hilbert space''' is a pair {{math|(''H'', Φ)}} with {{math|''H''}} a Hilbert space, {{math|Φ}} a dense subspace, such that {{math|Φ}} is given a topological vector space structure for which the inclusion map <math display="block">i : \Phi \to H,</math> is continuous.{{sfn|de la Madrid Modino|2001|pp=66-67}}{{sfn|van der Laan|2019|pp=21-22}} Identifying {{math|''H''}} with its dual space {{math|''H''*}}, the adjoint to {{math|''i''}} is the map <math display="block">i^* : H = H^* \to \Phi^*.</math>

The duality pairing between {{math|Φ}} and {{math|Φ*}} is then compatible with the inner product on {{math|''H''}}, in the sense that: <math display="block">\langle u, v\rangle_{\Phi\times\Phi^*} = (u, v)_H</math> whenever <math>u \in \Phi\subset H</math> and <math>v \in H = H^* \subset \Phi^*</math>. In the case of complex Hilbert spaces, we use a Hermitian inner product; it will be complex linear in {{math|''u''}} (math convention) or {{math|''v''}} (physics convention), and conjugate-linear (complex anti-linear) in the other variable.

The triple <math> (\Phi,\,\,H,\,\,\Phi^*)</math> is often named the ''Gelfand triple'' (after Israel Gelfand). <math>H</math> is referred to as a pivot space.

Note that even though {{math|Φ}} is isomorphic to {{math|Φ*}} (via Riesz representation) if it happens that {{math|Φ}} is a Hilbert space in its own right, this isomorphism is ''not'' the same as the composition of the inclusion {{math|''i''}} with its adjoint {{math|''i''*}} <math display="block">i^* i: \Phi\subset H = H^* \to \Phi^*.</math>

===Functional analysis approach===

The concept of rigged Hilbert space places this idea in an abstract functional-analytic framework. Formally, a rigged Hilbert space consists of a Hilbert space {{math|''H''}}, together with a subspace {{math|Φ}} which carries a finer topology, that is one for which the natural inclusion <math display="block"> \Phi \subseteq H </math> is continuous. It is no loss to assume that {{math|Φ}} is dense in {{math|''H''}} for the Hilbert norm. We consider the inclusion of dual spaces {{math|''H''*}} in {{math|Φ*}}. The latter, dual to {{math|Φ}} in its 'test function' topology, is realised as a space of distributions or generalised functions of some sort, and the linear functionals on the subspace {{math|Φ}} of type <math display="block">\phi\mapsto\langle v,\phi\rangle</math> for {{math|''v''}} in {{math|''H''}} are faithfully represented as distributions (because we assume {{math|Φ}} dense).

Now by applying the Riesz representation theorem we can identify {{math|''H''*}} with {{math|''H''}}. Therefore, the definition of ''rigged Hilbert space'' is in terms of a sandwich: <math display="block">\Phi \subseteq H \subseteq \Phi^*. </math>

The most significant examples are those for which {{math|Φ}} is a nuclear space; this comment is an abstract expression of the idea that {{math|Φ}} consists of test functions and {{math|Φ*}} of the corresponding distributions.

An example of a nuclear countably Hilbert space <math>\Phi</math> and its dual <math>\Phi^*</math> is the Schwartz space <math>\mathcal S(\mathbb R)</math> and the space of tempered distributions <math>\mathcal S'(\mathbb R)</math>, respectively, rigging the Hilbert space of square-integrable functions. As such, the rigged Hilbert space is given by{{sfn|de la Madrid Modino|2001|p=72}} <math display="block">\mathcal{S}(\mathbb{R}) \subset L^2 (\mathbb{R}) \subset \mathcal{S}'(\mathbb{R}).</math> Another example is given by Sobolev spaces: Here (in the simplest case of Sobolev spaces on <math>\mathbb R^n</math>) <math display="block">H = L^2(\mathbb R^n),\ \Phi = H^s(\mathbb R^n),\ \Phi^* = H^{-s}(\mathbb R^n),</math> where <math>s > 0</math>.

==Notes== {{reflist}}

==References== {{refbegin}} * J.-P. Antoine, ''Quantum Mechanics Beyond Hilbert Space'' (1996), appearing in ''Irreversibility and Causality, Semigroups and Rigged Hilbert Spaces'', Arno Bohm, Heinz-Dietrich Doebner, Piotr Kielanowski, eds., Springer-Verlag, {{isbn|3-540-64305-2}}. ''(Provides a survey overview.)'' * J. Dieudonné, ''Éléments d'analyse'' VII (1978). ''(See paragraphs 23.8 and 23.32)'' * {{cite book | last1=Gel'fand | first1=I. M. |author-link1=Israel Gelfand| last2=Vilenkin | first2=N. Ya | author-link2=Naum Yakovlevich Vilenkin|title=Generalized Functions: Applications of Harmonic Analysis | publisher=Elsevier Science | publication-place=Burlington | date=1964 | isbn=978-1-4832-2974-4 |doi=10.1016/c2013-0-12221-0}} * K. Maurin, ''Generalized Eigenfunction Expansions and Unitary Representations of Topological Groups'', Polish Scientific Publishers, Warsaw, 1968. * {{cite thesis |last=de la Madrid Modino |first= R. |date=2001 |title= Quantum mechanics in rigged Hilbert space language|url=https://scholar.google.com/scholar?oi=bibs&cluster=2442809273695897641&btnI=1&hl=en |degree= PhD |publisher= Universidad de Valladolid}} * de la Madrid Modino, R. "The role of the rigged Hilbert space in Quantum Mechanics," Eur. J. Phys. 26, 287 (2005); [https://arxiv.org/abs/quant-ph/0502053 quant-ph/0502053]. * {{cite thesis |last=van der Laan |first=L. |date= July 2019 |title=Rigged Hilbert Space Theory for Hermitian and Quasi-Hermitian Observables |url=https://fse.studenttheses.ub.rug.nl/19933/ |degree=BSc |location=Groningen |publisher=Rijksuniversiteit Groningen |access-date=11 January 2025}} {{refend}}

Category:Generalized functions Category:Hilbert spaces Category:Spectral theory Category:Schwartz distributions