In functional analysis, a branch of mathematics, a finite-rank operator is a bounded linear operator between Banach spaces whose range is finite-dimensional.[1]

Finite-rank operators on a Hilbert space

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A canonical form

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Finite-rank operators are matrices (of finite size) transplanted to the infinite dimensional setting. As such, these operators may be described via linear algebra techniques.

From linear algebra, we know that a rectangular matrix, with complex entries,   has rank   if and only if   is of the form

 

Exactly the same argument shows that an operator   on a Hilbert space   is of rank   if and only if

 

where the conditions on   are the same as in the finite dimensional case.

Therefore, by induction, an operator   of finite rank   takes the form

 

where   and   are orthonormal bases. Notice this is essentially a restatement of singular value decomposition. This can be said to be a canonical form of finite-rank operators.

Generalizing slightly, if   is now countably infinite and the sequence of positive numbers   accumulate only at  ,   is then a compact operator, and one has the canonical form for compact operators.

Compact operators are trace class only if the series   is convergent; a property that automatically holds for all finite-rank operators.[2]

Algebraic property

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The family of finite-rank operators   on a Hilbert space   form a two-sided *-ideal in  , the algebra of bounded operators on  . In fact it is the minimal element among such ideals, that is, any two-sided *-ideal   in   must contain the finite-rank operators. This is not hard to prove. Take a non-zero operator  , then   for some  . It suffices to have that for any  , the rank-1 operator   that maps   to   lies in  . Define   to be the rank-1 operator that maps   to  , and   analogously. Then

 

which means   is in   and this verifies the claim.

Some examples of two-sided *-ideals in   are the trace-class, Hilbert–Schmidt operators, and compact operators.   is dense in all three of these ideals, in their respective norms.

Since any two-sided ideal in   must contain  , the algebra   is simple if and only if it is finite dimensional.

Finite-rank operators on a Banach space

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A finite-rank operator   between Banach spaces is a bounded operator such that its range is finite dimensional. Just as in the Hilbert space case, it can be written in the form

 

where now  , and   are bounded linear functionals on the space  .

A bounded linear functional is a particular case of a finite-rank operator, namely of rank one.

References

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  1. ^ "Finite Rank Operator - an overview". 2004.
  2. ^ Conway, John B. (1990). A course in functional analysis. New York: Springer-Verlag. pp. 267–268. ISBN 978-0-387-97245-9. OCLC 21195908.