Projection-valued measure
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In mathematics, particularly functional analysis a projection-valued measure is a function defined on certain subsets of a fixed set and whose values are self-adjoint projections on a Hilbert space. Projection-valued measures are used to express results in spectral theory, such as the spectral theorem for self-adjoint operators.
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[edit] Formal definition
A projection-valued measure on a measurable space (X, M), where M is a σ-algebra of subsets of X, is a mapping π from M to the set of self-adjoint projections on a Hilbert space H such that
and for every ξ, η ∈ H, the set-function
is a complex measure on M (that is, a complex-valued countably additive function). We denote this measure by .
If π is a projection-valued measure and
then π(A), π(B) are orthogonal projections. From this follows that in general,
Example. Suppose (X, M, μ) is a measure space. Let π(A) be the operator of multiplication by the indicator function 1A on L2(X). Then π is a projection-valued measure.
[edit] Extensions of projection-valued measures
If π is an additive projection-valued measure on (X, M), then the map
extends to a linear map on the vector space of step functions on X. In fact, it is easy to check that this map is a ring homomorphism. In fact this map extends in a canonical way to all bounded complex-valued Borel functions on X.
Theorem. For any bounded M-measurable function f on X, there is a unique bounded linear operator Tπ(f) such that
for all ξ, η ∈ H. The map
is a homomorphism of rings.
[edit] Structure of projection-valued measures
First we provide a general example of projection-valued measure based on direct integrals. Suppose (X, M, μ) is a measure space and let {Hx}x ∈ X be a μ-measurable family of separable Hilbert spaces. For every A ∈ M, let π(A) be the operator of multiplication by 1A on the Hilbert space
Then π is a projection-valued measure on (X, M).
Suppose π, ρ are projection-valued measures on (X, M) with values in the projections of H, K. π, ρ are unitarily equivalent if and only if there is a unitary operator U:H → K such that
for every A ∈ M.
Theorem. If (X, M) is a standard Borel space, then for every projection-valued measure π on (X, M) taking values in the projections of a separable Hilbert space, there is a Borel measure μ and a μ-measurable family of Hilbert spaces {Hx}x ∈ X , such that π is unitarily equivalent to multiplication by 1A on the Hilbert space
The measure class of μ and the measure equivalence class of the multiplicity function x → dim Hx completely characterize the projection-valued measure up to unitary equivalence.
A projection-valued measure π is homogeneous of multiplicity n if and only if the multiplicity function has constant value n. Clearly,
Theorem. Any projection-valued measure π taking values in the projections of a separable Hilbert space is an orthogonal direct sum of homogeneous projection-valued measures:
where
and
[edit] Generalizations
The idea of a projection-valued measure is generalized by the positive operator-valued measure, where the need for the orthogonality implied by projection operators is replaced by the idea of a set of operators that are a non-orthogonal partition of unity.
[edit] References
- G. W. Mackey, The Theory of Unitary Group Representations, The University of Chicago Press, 1976
- V. S. Varadarajan, Geometry of Quantum Theory V2, Springer Verlag, 1970.