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is finite. In this case, the sum
is absolutely convergent and is independent of the choice of the orthonormal basis. This value is called the trace of A, denoted by Tr(A).
By extension, if A is a non-negative self-adjoint operator, we can also define the trace of A as an extended real number by the possibly divergent sum
If A is a non-negative self-adjoint, A is trace class iff Tr(A) < ∞. An operator A is trace class iff its positive part A+ and negative part A- are both trace class.
The trace is a linear functional over the space of trace class operators, meaning
The bilinear map
is an inner product on the trace class; the corresponding norm is called the Hilbert-Schmidt norm. The completion of the trace class operators in the Hilbert-Schmidt norm can also be considered as a class of operators, the Hilbert-Schmidt operators.
For infinite dimensional spaces, the class of Hilbert-Schmidt operators is strictly larger than that of trace class operators. The heuristic is that Hilbert-Schmidt is to trace class as l2(N) is to l1(N).
The set C1 of trace class operators on H is a two-sided ideal in B(H), the set of all bounded linear operators on H. So given any operator T in B(H), we may define a continuous linear functional φT on C1 by φT(A)=Tr(AT). This correspondence between elements φT of the dual space of C1 and bounded linear operators is an isometric isomorphism. It follows that B(H) is the dual space of C1. This can be used to defined the weak-* topology on B(H).
J. Dixmier, Les Algebres d'Operateurs dans l'Espace Hilbertien, Gauthier-Villars, 1969
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