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There have been three main ways to formulate spectral theory, all of which retain their usefulness. After Hilbert's initial formulation, the later development of abstract Hilbert Space and the spectral theory of a single Normal Operator on it did very much go in parallel with the requirements of Physics ; particularly at the hands of Von Neumann . The further theory built on this to include Banach Algebra s, which can be given abstractly. This development leads to the Gelfand Representation , which covers the Commutative Case , and further into Non-commutative Harmonic Analysis . The difference can be seen in making the connection with Fourier Analysis . The Fourier Transform on the Real Line is in one sense the spectral theory of Differentiation ''qua'' Differential Operator . But for that to cover the phenomena one has already to deal with Generalized Eigenfunction s (for example, by means of a Rigged Hilbert Space ). On the other hand it is simple to construct a Group Algebra , the spectrum of which captures the Fourier transform's basic properties, and this is carried out by means of Pontryagin Duality . One can also study the spectral properties of operators on Banach Spaces . For example, Compact Operator s on Banach spaces have many spectral properties similar to that of Matrices . Aspects of spectral theory include:
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