# The Backward Shift on the Hardy Space by Joseph A. Cima

By Joseph A. Cima

Shift operators on Hilbert areas of analytic capabilities play a massive function within the examine of bounded linear operators on Hilbert areas because they typically function versions for varied periods of linear operators. for instance, parts'' of direct sums of the backward shift operator at the classical Hardy house $H^2$ version specific sorts of contraction operators and in all probability have connections to realizing the invariant subspaces of a common linear operator. This e-book is an intensive therapy of the characterization of the backward shift invariant subspaces of the well known Hardy areas $H^{p}$. The characterization of the backward shift invariant subspaces of $H^{p}$ for $1 < p < \infty$ was once performed in a 1970 paper of R. Douglas, H. S. Shapiro, and A. Shields, and the case $0 < p \le 1$ used to be performed in a 1979 paper of A. B. Aleksandrov which isn't popular within the West. This fabric is pulled jointly during this unmarried quantity and contains all of the priceless historical past fabric had to comprehend (especially for the $0 < p < 1$ case) the proofs of those effects. a number of proofs of the Douglas-Shapiro-Shields consequence are supplied so readers can get familiar with diverse operator concept and thought thoughts: functions of those proofs also are supplied for knowing the backward shift operator on a variety of different areas of analytic capabilities. the implications are completely tested. different beneficial properties of the amount contain an outline of functions to the spectral houses of the backward shift operator and a remedy of a few basic real-variable suggestions that aren't taught in usual graduate seminars. The ebook comprises references to works via Duren, Garnett, and Stein for proofs and a bibliography for extra exploration within the parts of operator concept and sensible research.

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Extra resources for The Backward Shift on the Hardy Space

Example text

A simple application of Holder's inequality shows that the linear functional tg(f) •= J f9 dm is continuous on Hp whenever g belongs to Hq. Conversely, by the Hahn-Banach extension theorem, any continuous linear functional £ on Hp can be extended to a continuous linear functional on Lp and so, by the Riesz representation theorem, £(f) = j fgdm feHp for some g G Lq. For trigonometric polynomials p\ and p2, a simple computation reveals that / (Ppi)p2 dm= P1PP2 dm. 3. 5, and the density of the trigonometric polynomials in Lp, we can replace p\ and p2 with arbitrary functions in Lp (respectively Lq).

And R. 1. 2. The Smirnov class. Although (iV, p) is not a topological vector space, there is a natural closed subalgebra of N which is. Recall that feN^ lim / l o g ( l + |/(rC)|)dm(C)

1) is meaningful and even has several desirable properties. By this identification of HP{C\R) with HP(R), we can view Ep(F,k) as the distributions £ G HP(R) such that 1. lim £ ( — ) = lim £ ( y^o+ v • - (x + iy) ' y-^o- v '-(x almost everywhere. 2. suppt £c F. 3. ci6*^ 3=0 where Ux is any interval about x which does not contain any other point of F except x. 2. 1), is Mz)= e i i i 2ri (—Z)=2ri N k Xn ( "> \ E E (J-^i+i n—1 j—U which is indeed a rational function. Why this desire to view everything in terms of distributions?