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A Caratheodory theorem for the bidisk via Hilbert space methods
If \ph is an analytic function bounded by 1 on the bidisk \D^2 and
\tau\in\tb is a point at which \ph has an angular gradient
\nabla\ph(\tau) then \nabla\ph(\la) \to \nabla\ph(\tau) as \la\to\tau
nontangentially in \D^2. This is an analog for the bidisk of a classical
theorem of Carath\'eodory for the disk.
For \ph as above, if \tau\in\tb is such that the of
(1-|\ph(\la)|)/(1-\|\la\|) as \la\to\tau is finite then the directional
derivative D_{-\de}\ph(\tau) exists for all appropriate directions
\de\in\C^2. Moreover, one can associate with \ph and an analytic
function in the Pick class such that the value of the directional
derivative can be expressed in terms of
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