Toeplitz operator

Let S¹ be the circle, with the standard Lebesgue measure, and L²(S¹) be the Hilbert space of square-integrable functions. A bounded measurable function g on S¹ defines a multiplication operator M_g on L²(S¹). Let P be the projection from L²(S¹) onto the Hardy space H². The Toeplitz operator with symbol g is defined by

${\displaystyle T_{g}=PM_{g}\vert _{H^{2}},}$

where " | " means restriction.

A bounded operator on H² is Toeplitz if and only if its matrix representation, in the basis {zⁿ, n ≥ 0}, has constant diagonals.

• Theorem: If ${\displaystyle g}$ is continuous, then ${\displaystyle T_{g}-\lambda }$ is Fredholm if and only if ${\displaystyle \lambda }$ is not in the set ${\displaystyle g(S^{1})}$. If it is Fredholm, its index is minus the winding number of the curve traced out by ${\displaystyle g}$ with respect to the origin.

For a proof, see Douglas (1972, p.185). He attributes the theorem to Mark Krein, Harold Widom and Allen Devinatz. This can be thought of as an important special case of the Atiyah-Singer index theorem.

• Axler- Chang- Sarason Theorem: The operator ${\displaystyle T_{f}T_{g}-T_{fg}}$ is compact if and only if ${\displaystyle H^{\infty }[{\bar {f}}]\cap H^{\infty }[g]\subseteq H^{\infty }+C}$.

Here, ${\displaystyle H^{\infty }}$ denotes the closed subalgebra of ${\displaystyle L^{\infty }(S^{1})}$ of analytic functions (functions with vanishing negative Fourier coefficients), ${\displaystyle H^{\infty }[f]}$ is the closed subalgebra of ${\displaystyle L^{\infty }(S^{1})}$ generated by ${\displaystyle f}$ and ${\displaystyle H^{\infty }}$, and ${\displaystyle C}$ is the continuous functions on the circle. See S.Axler, S-Y. Chang, D. Sarason (1978)

• S.Axler, S-Y. Chang, D. Sarason (1978), "Products of Toeplitz operators", Integral Equations and Operator Theory, 1: 285–309CS1 maint: multiple names: authors list (link)
• Böttcher, Albrecht; Grudsky, Sergei M. (2000), Toeplitz Matrices, Asymptotic Linear Algebra, and Functional Analysis, Birkhäuser, ISBN 978-3-0348-8395-5.
• Böttcher, A.; Silbermann, B. (2006), Analysis of Toeplitz Operators, Springer Monographs in Mathematics (2nd ed.), Springer-Verlag, ISBN 978-3-540-32434-8.
• Douglas, Ronald (1972), Banach Algebra techniques in Operator theory, Academic Press.
• Rosenblum, Marvin; Rovnyak, James (1985), Hardy Classes and Operator Theory, Oxford University Press. Reprinted by Dover Publications, 1997, ISBN 978-0-486-69536-5.