%N Wechsler's "convex-hull polyominoes": convex hull contains no additional grid points.
%C Given a polyomino P on a square lattice, if you replace each of the squares in P with a point (say the "upper-left" corner) and call that set of points S, then define H to be the convex hull of S: the polyomino is said to be a "convex-hull polyomino" if all lattice points in H are also in S.
%H R. Munafo, <a href="http://mrob.com/pub/math/seq-a181785.html">Wechsler's Convex-Hull Polyominoes</a>
%e For N=5 there are 12 polyominoes, but only 10 qualify. The two that do not are the "U" and "V" pentominoes, pictured here:
%e . * . * . . . * * *
%e . * * * . . . * . .
%e . . . . . . . * . .
%e Both are "concave" in the sense that a convex hull of the 5 points in the pentomino also includes one grid point that is not in the pentomino.
%Y Cf. A000105
%A _Robert Munafo_, May 08 2011
%E Initial entry by _Robert Munafo_, May 08 2011
%E Name changed (with Wechsler's approval) by _Robert Munafo_, May 12 2011
%E a(14)-a(16) added by _Robert Munafo_, May 12 2011