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 A181785 Wechsler's "convex-hull polyominoes": convex hull contains no additional grid points. 1
 1, 1, 2, 5, 10, 25, 48, 107, 193, 365, 621, 1082, 1715, 2777, 4247, 6519 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,3 COMMENTS 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. LINKS R. Munafo, Wechsler's Convex-Hull Polyominoes EXAMPLE For N=5 there are 12 polyominoes, but only 10 qualify. The two that do not are the "U" and "V" pentominoes, pictured here: . * . * . . . * * * . * * * . . . * . . . . . . . . . * . . 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. CROSSREFS Cf. A000105 Sequence in context: A001431 A054866 A109616 * A018262 A018356 A026383 Adjacent sequences:  A181782 A181783 A181784 * A181786 A181787 A181788 KEYWORD nonn,hard AUTHOR Robert Munafo, May 08 2011 EXTENSIONS Initial entry by Robert Munafo, May 08 2011 Name changed (with Wechsler's approval) by Robert Munafo, May 12 2011 a(14)-a(16) added by Robert Munafo, May 12 2011 STATUS approved

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Last modified December 17 06:45 EST 2018. Contains 318192 sequences. (Running on oeis4.)