Helson-Szegö Measure

An absolutely continuous measure on $\partial D$ whose density has the form $\mathop{\rm exp}\nolimits (x+\bar y)$, where $x$ and $y$ are real-valued functions in $L^\infty$, $\vert\vert y\vert\vert _\infty<\pi/2$, exp is the Exponential Function, and $\vert\vert y\vert\vert$ is the Norm.