A131481 a(n) is the number of n-cell polyiamonds (triangular polyominoes) with perimeter n+2.

1, 1, 1, 3, 4, 11, 23, 62, 149, 409, 1066, 2931, 7981, 22166, 61508, 172267, 483088, 1361475, 3845139, 10894630

Offset

1,4

Comments

n+2 is the maximal perimeter for an n-celled polyiamond. a(n) is the number of n-celled polyiamonds that have a tree as their connectedness graph (vertices of this graph correspond to cells and two vertices are connected if the corresponding cells have a common edge)

Links

Table of n, a(n) for n=1..20.

Andrew Clarke, Isoperimetrical Polyiamonds

Crossrefs

a(n) <= A000577(n), a(n) <= A057729(n+2).

Sequence in context: A247171 A005218 A219514 * A001072 A292623 A077900

Adjacent sequences: A131478 A131479 A131480 * A131482 A131483 A131484

Keyword

nonn,hard,more

Author

Tanya Khovanova, Jul 27 2007

Extensions

Offset corrected and terms a(17)-a(20) added by John Mason, May 15 2021

Status

approved