Approximate identity

Approximate identity

In functional analysis, a right approximate identity in a Banach algebra, "A", is a net (or a sequence)

:{,e_lambda : lambda in Lambda,}

such that for every element, "a", of "A", the net (or sequence)

:{,ae_lambda:lambda in Lambda,}

has limit "a".

Similarly, a left approximate identity is a net

:{,e_lambda : lambda in Lambda,}

such that for every element, "a", of "A", the net (or sequence)

:{,e_lambda a: lambda in Lambda,}

has limit "a".

An approximate identity is a right approximate identity which is also a left approximate identity.

For C*-algebras, a right (or left) approximate identity is the same as an approximate identity. Every C*-algebra has an approximate identity of positive elements of norm ≤ 1; indeed, the net of all positive elements of norm ≤ 1; in "A" with its natural order always suffices. This is called the canonical approximate identity of a C*-algebra. Approximate identities of C*-algebras are not unique. For example, for compact operators acting on a Hilbert space, the net consisting of finite rank projections would be another approximate identity.

An approximate identity in a convolution algebra plays the same role as a sequence of function approximations to the Dirac delta function (which is the identity element for convolution). For example the Fejér kernels of Fourier series theory give rise to an approximate identity.

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