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This article is about the mathematical operator; it is not to be confused with Reactor operator.

In mathematics, nuclear operators are an important class of linear operators introduced by Alexander Grothendieck in his doctoral dissertation. Nuclear operators are intimately tied to the projective tensor product of two topological vector spaces (TVSs).

Throughout let X,Y, and Z be topological vector spaces (TVSs) and L : X → Y be a linear operator (no assumption of continuity is made unless otherwise stated).

• The projective tensor product of two locally convex TVSs X and Y is denoted by and the completion of this space will be denoted by .
• L : X → Y is a topological homomorphism or homomorphism, if it is linear, continuous, and is an open map, where , the image of L, has the subspace topology induced by Y.
  • If S is a subspace of X then both the quotient map X → X/S and the canonical injection S → X are homomorphisms.
• The set of continuous linear maps X → Z (resp. continuous bilinear maps ) will be denoted by L(X, Z) (resp. B(X, Y; Z)) where if Z is the underlying scalar field then we may instead write L(X) (resp. B(X, Y)).
• Any linear map can be canonically decomposed as follows: where defines a bijection called the canonical bijection associated with L.
• X* or will denote the continuous dual space of X.
• will denote the algebraic dual space of X (which is the vector space of all linear functionals on X, whether continuous or not).
• A linear map L : H → H from a Hilbert space into itself is called positive if for every . In this case, there is a unique positive map r : H → H, called the square-root of L, such that .^[1]
• A linear map is called compact or completely continuous if there is a neighborhood U of the origin in X such that is precompact in Y.^[2]

In a Hilbert space, positive compact linear operators, say L : H → H have a simple spectral decomposition discovered at the beginning of the 20th century by Fredholm and F. Riesz:^[3]

There is a sequence of positive numbers, decreasing and either finite or else converging to 0, and a sequence of nonzero finite dimensional subspaces of H (i = 1, 2, ) with the following properties: (1) the subspaces are pairwise orthogonal; (2) for every i and every , ; and (3) the orthogonal of the subspace spanned by is equal to the kernel of L.^[3]

Let X and Y be vector spaces (no topology is needed yet) and let Bi(X, Y) be the space of all bilinear maps defined on and going into the underlying scalar field.

For every , let be the canonical linear form on Bi(X, Y) defined by for every u ∈ Bi(X, Y). This induces a canonical map defined by , where denotes the algebraic dual of Bi(X, Y). If we denote the span of the range of 𝜒 by X ⊗ Y then it can be shown that X ⊗ Y together with 𝜒 forms a tensor product of X and Y (where x ⊗ y := 𝜒(x, y)). This gives us a canonical tensor product of X and Y.

If Z is any other vector space then the mapping Li(X ⊗ Y; Z) → Bi(X, Y; Z) given by u ↦ u ∘ 𝜒 is an isomorphism of vector spaces. In particular, this allows us to identify the algebraic dual of X ⊗ Y with the space of bilinear forms on X × Y.^[4] Moreover, if X and Y are locally convex topological vector spaces (TVSs) and if X ⊗ Y is given the π-topology then for every locally convex TVS Z, this map restricts to a vector space isomorphism from the space of continuous linear mappings onto the space of continuous bilinear mappings.^[5] In particular, the continuous dual of X ⊗ Y can be canonically identified with the space B(X, Y) of continuous bilinear forms on X × Y; furthermore, under this identification the equicontinuous subsets of B(X, Y) are the same as the equicontinuous subsets of .^[5]

There is a canonical vector space embedding defined by sending to the map

Assuming that X and Y are Banach spaces, then the map has norm (to see that the norm is , note that so that ). Thus it has a continuous extension to a map , where it is known that this map is not necessarily injective.^[6] The range of this map is denoted by and its elements are called nuclear operators.^[7] is TVS-isomorphic to and the norm on this quotient space, when transferred to elements of via the induced map , is called the trace-norm and is denoted by . Explicitly,^{[clarification needed explicitly or especially?]} if is a nuclear operator then .

Suppose that X and Y are Banach spaces and that is a continuous linear operator.

Let X and Y be Banach spaces and let be a continuous linear operator.

Nuclear automorphisms of a Hilbert space are called trace class operators.

Let X and Y be Hilbert spaces and let N : X → Y be a continuous linear map. Suppose that where R : X → X is the square-root of and U : X → Y is such that is a surjective isometry. Then N is a nuclear map if and only if R is a nuclear map; hence, to study nuclear maps between Hilbert spaces it suffices to restrict one's attention to positive self-adjoint operators R.^[11]

Let X and Y be Hilbert spaces and let N : X → Y be a continuous linear map whose absolute value is R : X → X. The following are equivalent:

1. N : X → Y is nuclear.
2. R : X → X is nuclear.^[12]
3. R : X → X is compact and is finite, in which case .^[12]
4. is nuclear, in which case . ^[9]
5. There are two orthogonal sequences in X and in Y, and a sequence in such that for all , .^[12]
6. N : X → Y is an integral map.^[13]

Suppose that U is a convex balanced closed neighborhood of the origin in X and B is a convex balanced bounded Banach disk in Y with both X and Y locally convex spaces. Let and let be the canonical projection. One can define the auxiliary Banach space with the canonical map whose image, , is dense in as well as the auxiliary space normed by and with a canonical map being the (continuous) canonical injection. Given any continuous linear map one obtains through composition the continuous linear map ; thus we have an injection and we henceforth use this map to identify as a subspace of .^[7]

Definition: Let X and Y be Hausdorff locally convex spaces. The union of all as U ranges over all closed convex balanced neighborhoods of the origin in X and B ranges over all bounded Banach disks in Y, is denoted by and its elements are call nuclear mappings of X into Y.^[7]

When X and Y are Banach spaces, then this new definition of nuclear mapping is consistent with the original one given for the special case where X and Y are Banach spaces.

• Let W, X, Y, and Z be Hausdorff locally convex spaces, a nuclear map, and and be continuous linear maps. Then , , and are nuclear and if in addition W, X, Y, and Z are all Banach spaces then .^[14][15]
• If is a nuclear map between two Hausdorff locally convex spaces, then its transpose is a continuous nuclear map (when the dual spaces carry their strong dual topologies).^[2]
  • If in addition X and Y are Banach spaces, then .^[9]
• If is a nuclear map between two Hausdorff locally convex spaces and if is a completion of X, then the unique continuous extension of N is nuclear.^[15]

Let X and Y be Hausdorff locally convex spaces and let be a continuous linear operator.

The following is a type of Hahn-Banach theorem for extending nuclear maps:

Let X and Y be Hausdorff locally convex spaces and let be a continuous linear operator.

• Auxiliary normed spaces
• Covariance operator – Operator in probability theory
• Initial topology – Coarsest topology making certain functions continuous
• Inductive tensor product
• Injective tensor product
• Locally convex topological vector space – Space with topology generated by convex sets
• Nuclear operators between Banach spaces
• Nuclear space – Generalization of finite-dimensional Euclidean spaces different from Hilbert spaces
• Projective tensor product
• Tensor product of Hilbert spaces – Tensor product space endowed with a special inner product
• Topological tensor product – Tensor product constructions for topological vector spaces
• Trace class – Compact operator for which a finite trace can be defined
• Topological vector space – Vector space with a notion of nearness

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• Nuclear space at ncatlab