In mathematics, a nuclear operator is a compact operator for which a trace may be defined, such that the trace is finite and independent of the choice of basis (at least on well behaved spaces; there are some spaces on which nuclear operators do not have a trace). Nuclear operators are essentially the same as trace class operators, though most authors reserve the term "trace class operator" for the special case of nuclear operators on Hilbert spaces.
The general definition for Banach spaces was given by Grothendieck. This article presents both cases but concentrates on the general case of nuclear operators on Banach spaces; for more details about the important special case of nuclear (= trace class) operators on Hilbert space see the article on trace class operators.
An operator L on a Hilbert space H
where 1 ≤ N ≤ ∞ and and are (not necessarily complete) orthonormal sets. Here, ρ1, ... ,ρN are a set of real numbers, the singular values of the operator, obeying ρn → 0 if N = ∞.
The bracket is the scalar product on the Hilbert space; the sum on the right hand side must converge in norm.
An operator that is compact as defined above is said to be nuclear or trace-class if
A nuclear operator on a Hilbert space has the important property that a trace operation may be defined. Given an orthonormal basis for the Hilbert space, the trace is defined as
It is immediate the sum converges absolutely, and it can be proven that the result is independent of the basis. It can be shown that this trace is identical to the sum of the eigenvalues of (counted with multiplicity).
On Banach spaces
- See main article Fredholm kernel.
(from the projective tensor product of A' and B to the Banach space of continuous linear maps from A to B). It is determined by sending and to the linear map . An operator is called nuclear if it is in the image of this evaluation map.
is said to be nuclear of order q if there exist sequences of vectors with , functionals with and complex numbers with
such that the operator may be written as
with the sum converging in the operator norm.
Operators that are nuclear of order 1 are called nuclear operators: these are the ones for which the series ∑ρn is absolutely convergent. Nuclear operators of order 2 are called Hilbert–Schmidt operators.
Relation to trace class operators
With additional steps, a trace may be defined for such operators when A = B.
More generally, an operator from a locally convex topological vector space A to a Banach space B is called nuclear if it satisfies the condition above with all fn* bounded by 1 on some fixed neighborhood of 0.
An extension of the concept of nuclear maps to arbitrary monoidal categories is given by Stolz & Teichner (2012). A monoidal category can be thought of as a category equipped with a suitable notion of a tensor product. An example of a monoidal category is the category of Banach spaces or alternatively the category of locally convex, complete, Hausdorff spaces; both equipped with the projective tensor product. A map in a monoidal category is called thick if it can be written as a composition
for an appropriate object C and maps , where I is the monoidal unit.
In the monoidal category of Banach spaces, equipped with the projective tensor product, a map is thick if and only if it is nuclear.
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- Stolz, Stephan; Teichner, Peter (2012), "Traces in monoidal categories", Transactions of the American Mathematical Society, 364 (8): 4425–4464, arXiv: , doi:10.1090/S0002-9947-2012-05615-7, MR 2912459