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Fundamental theorem of Hilbert spaces

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In mathematics, specifically in functional analysis and Hilbert space theory, the fundamental theorem of Hilbert spaces gives a necessarily and sufficient condition for a Hausdorff pre-Hilbert space to be a Hilbert space in terms of the canonical isometry of a pre-Hilbert space into its anti-dual.

Preliminaries

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Antilinear functionals and the anti-dual

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Suppose that H is a topological vector space (TVS). A function f : H is called semilinear or antilinear[1] if for all x, yH and all scalars c ,

The vector space of all continuous antilinear functions on H is called the anti-dual space or complex conjugate dual space of H and is denoted by (in contrast, the continuous dual space of H is denoted by ), which we make into a normed space by endowing it with the canonical norm (defined in the same way as the canonical norm on the continuous dual space of H).[1]

Pre-Hilbert spaces and sesquilinear forms

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A sesquilinear form is a map B : H × H such that for all yH, the map defined by xB(x, y) is linear, and for all xH, the map defined by yB(x, y) is antilinear.[1] Note that in Physics, the convention is that a sesquilinear form is linear in its second coordinate and antilinear in its first coordinate.

A sesquilinear form on H is called positive definite if B(x, x) > 0 for all non-0 xH; it is called non-negative if B(x, x) ≥ 0 for all xH.[1] A sesquilinear form B on H is called a Hermitian form if in addition it has the property that for all x, yH.[1]

Pre-Hilbert and Hilbert spaces

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A pre-Hilbert space is a pair consisting of a vector space H and a non-negative sesquilinear form B on H; if in addition this sesquilinear form B is positive definite then (H, B) is called a Hausdorff pre-Hilbert space.[1] If B is non-negative then it induces a canonical seminorm on H, denoted by , defined by xB(x, x)1/2, where if B is also positive definite then this map is a norm.[1] This canonical semi-norm makes every pre-Hilbert space into a seminormed space and every Hausdorff pre-Hilbert space into a normed space. The sesquilinear form B : H × H is separately uniformly continuous in each of its two arguments and hence can be extended to a separately continuous sesquilinear form on the completion of H; if H is Hausdorff then this completion is a Hilbert space.[1] A Hausdorff pre-Hilbert space that is complete is called a Hilbert space.

Canonical map into the anti-dual

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Suppose (H, B) is a pre-Hilbert space. If hH, we define the canonical maps:

B(h, •) : H       where       yB(h, y),     and
B(•, h) : H       where       xB(x, h)

The canonical map[1] from H into its anti-dual is the map

      defined by       xB(x, •).

If (H, B) is a pre-Hilbert space then this canonical map is linear and continuous; this map is an isometry onto a vector subspace of the anti-dual if and only if (H, B) is a Hausdorff pre-Hilbert.[1]

There is of course a canonical antilinear surjective isometry that sends a continuous linear functional f on H to the continuous antilinear functional denoted by f and defined by xf (x).

Fundamental theorem

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Fundamental theorem of Hilbert spaces:[1] Suppose that (H, B) is a Hausdorff pre-Hilbert space where B : H × H is a sesquilinear form that is linear in its first coordinate and antilinear in its second coordinate. Then the canonical linear mapping from H into the anti-dual space of H is surjective if and only if (H, B) is a Hilbert space, in which case the canonical map is a surjective isometry of H onto its anti-dual.

See also

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References

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  1. ^ a b c d e f g h i j k Trèves 2006, pp. 112–123.
  • Narici, Lawrence; Beckenstein, Edward (2011). Topological Vector Spaces. Pure and applied mathematics (Second ed.). Boca Raton, FL: CRC Press. ISBN 978-1584888666. OCLC 144216834.
  • Schaefer, Helmut H.; Wolff, Manfred P. (1999). Topological Vector Spaces. GTM. Vol. 8 (Second ed.). New York, NY: Springer New York Imprint Springer. ISBN 978-1-4612-7155-0. OCLC 840278135.
  • Trèves, François (2006) [1967]. Topological Vector Spaces, Distributions and Kernels. Mineola, N.Y.: Dover Publications. ISBN 978-0-486-45352-1. OCLC 853623322.