Back to Boundaries in Billiards

We prove Poisson limit laws for open billiard systems with holes in the boundary of billiard tables. Traditionally some abstract holes in the phase space of a billiard were studied. Holes in the boundary are of an intrinsic interest for billiard systems, especially for applications. Sinai billiards with or without a finite horizon, diamond billiards, and semi-dispersing billiards are considered. However, the emphasis is on focusing billiards with slow decay of correlations, where various new technical difficulties arise.