= Hyperbolic geodesic cap
{title2=$Q(I)$}
For a proper arc $I$ of the boundary of the <unit disc>, $Q(I)$ is the side of the <hyperbolic geodesic> between its endpoints adjacent to $I$. For short arcs this is a curved boundary cap. For arcs longer than a semicircle it contains the center. With endpoints $e^{\pm i\alpha}$ and $0<\alpha<\pi/2$, the geodesic is the circle with center $\sec\alpha$ and radius $\tan\alpha$, orthogonal to the <unit circle>. It is a diameter at $\alpha=\pi/2$.
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