Image of repeating wallpaper with only 3-fold rotational symmetries versus image of repeating wallpaper with some other symmetries.
Left examples have wallpaper symmetry group p3 (a.k.a. 333).
Aaron David Fairbanks
Bongard Problems with "world" spiraling curves vs. other Bongard Problems.
Non-self-intersecting curves with turning number greater than one.
