2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/148572It is proven that, under some conditions on $f$, the non-compact flat billiard $Ω= \{ (x,y) \in \R_0^{+} \times \R_0^{+};\ 0\le y \le f(x) \}$ has no orbits going {\em directly} to $+\infty$. The relevance of such sufficient conditions is discussed.9 pages, LaTeX, 3 postscript figures available at http://www.princeton.edu/~marco/papers/ . Minor changes since previously posted version. Submitted to 'Chaos'Chaotic Dynamicstext