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Weakly Periodic Sequences of Bounded Linear Transformations: a Spectral Characterization
Authors:Soltani  A R  Shishebor  Z
Institution:(1) Center for Theoretical Physics and Mathematics AEOI, P.O. Box 11365-8486, Tehran, Iran;(2) Department of Statistics, Shiraz University, Shiraz, 71454, Iran
Abstract:Let X and Y be two Hilbert spaces, and 
$$\mathcal{L}(X,Y)$$
the space of bounded linear transformations from X into Y. Let {A eegr} sub 
$$\mathcal{L}(X,Y)$$
be a weakly periodic sequence of period T. Spectral theory of weakly periodic sequences in a Hilbert space is studied by H. L. Hurd and V. Mandrekar (1991). In this work we proceed further to characterize {A n} by a positive measure mgr and a number T of 
$$\mathcal{L}(X,X)$$
-valued functions a 0, . . . ,a T–1; in the spectral form 
$$A_n = \smallint _0^{2\pi } e^{ - i\lambda n} \Phi (d\lambda )Vn(\lambda )$$
, where 
$$Vn(\lambda ) = \sum\nolimits_{k = 0}^{T - 1} {e^{ - i\frac{{2\pi kn}}{T}} a_k } (\lambda )$$
and PHgr is an 
$$\mathcal{L}(X,Y)$$
-valued Borel set function on 0, 2pgr) such that 
$$(\Phi (\Delta )x, \Phi (\Delta ')x')_Y = (x,x')_X \mu (\Delta \cap \Delta ').$$
Keywords:Hilbert space  bounded linear operators  weakly periodic sequences  spectral representation
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