On Jan 16, 2:34 pm, quasi <qu...@null.set> wrote: > [. . .] > U is star-shaped in the sense that any point in U is "visible" > from (0,0). > > Thus, U need not be a disk. > > For example, any convex open set containing (0,0) qualifies. > > But U need not be convex either. > > For example, the interior of a Jewish star centered at (0,0) > also qualifies.
> Staying with the religious theme, the interior of a Christian > cross centered at (0,0) qualifies as well.
I have my doubts about that. I guess it depends on how you define the "center" of a Christian cross.