Unbounded operators #
i. Overview #
In this module we introduce unbounded operators on inner product spaces. By "unbounded operator"
we mean a partially-defined linear map (LinearPMap) which is both densely defined and closable.
These provide the mathematical structure appropriate for describing operators in non-relativistic
quantum mechanics. Of particular interest are (essentially) self-adjoint unbounded operators since
these correspond to physical observables.
Notes #
Naming convention : Definitions of
LinearPMaps for quantum mechanical unbounded operators should have a name of the form[β¦]Operatorand notation should use calligraphic capital letters, e.g.mulOperator f(π f) for the multiplication operator associated with the functionf.Implementation : Although operators encountered in quantum mechanics are almost always unbounded, we opt to implement unbounded operators via the property
IsUnboundedonLinearPMaprather than as a structureUnboundedOperatorextendingLinearPMap. The basic reason for this is that addition/subtraction and composition of unbounded operators in general does not result in another unbounded operator. This means, for example, that any attempt to define addition ofUnboundedOperators would inevitably require introducing junk values that spoil associativity.
ii. Key results #
adjoint_add_le_add_adjoint: The inequalityUββ + Uββ β€ (Uβ + Uβ)βwhenUβ + Uβhas dense domain.IsEssentiallySelfAdjoint.unique_self_adjoint_extension: The closure of an essentially self-adjoint unbounded operator is its unique self-adjoint extension.IsUnbounded.adjoint: The adjoint of an unbounded operator is also unbounded.IsUnbounded.adjoint_closure_eq_adjoint: An unbounded operator and its closure have the same adjoint.IsUnbounded.adjoint_adjoint_eq_closure: An unbounded operatorUsatisfiesUβ β = U.closure.
iii. Table of contents #
- A. Definitions
- B. Dense domain
- C. Closability
- D. Adjoints
- E. Symmetric operators
- F. Self-adjoint operators
- G. Essentially self-adjoint operators
- H. Unbounded operators
iv. References #
- M. Reed & B. Simon, (1972). "Methods of modern mathematical physics: Vol. 1. Functional analysis". Academic Press. https://doi.org/10.1016/B978-0-12-585001-8.X5001-6
- K. SchmΓΌdgen, (2012). "Unbounded self-adjoint operators on Hilbert space" (Vol. 265). Springer. https://doi.org/10.1007/978-94-007-4753-1
A. Definitions #
See LinearPMap.instStar and LinearPMap.isSelfAdjoint_def for the definition of IsSelfAdjoint
for LinearPMaps.
A LinearPMap U has dense domain iff U.domain is dense in H.
Equations
- U.HasDenseDomain = Dense βU.domain
Instances For
A LinearPMap is an unbounded operator iff it has dense domain and is closable.
Equations
- U.IsUnbounded = (U.HasDenseDomain β§ U.IsClosable)
Instances For
A LinearPMap U is symmetric iff βͺU x, yβ«_β = βͺx, U yβ«_β for all x y : U.domain.
Equations
- T.IsSymmetric = T.IsFormalAdjoint T
Instances For
A LinearPMap is essentially self-adjoint iff its closure is self-adjoint.
Equations
Instances For
B. Dense domain #
C. Closability #
A LinearPMap with densely-defined formal adjoint is closable.
A zero LinearPMap (any domain) is closable.
D. Adjoints #
The adjoint of a zero LinearPMap (any domain) is zero (domain β€).
E. Symmetric operators #
The analogue of inner_map_polarization for LinearPMap.
The analogue of inner_map_polarization' for LinearPMap.
F. Self-adjoint operators #
G. Essentially self-adjoint operators #
The closure is the unique self-adjoint extension of an essentially self-adjoint operator.
H. Unbounded operators #
A LinearPMap constructed from a symmetric LinearMap with dense domain is an unbounded operator.
Variant of of_dense_of_isSymmetric for an endomorphism satisfying LinearMap.IsSymmetric.