A122672 Number of primitive coronoid systems with n hexagons.

1, 1, 3, 2, 11, 12, 40, 68, 192, 395, 1061, 2432, 6318, 15417, 39660, 99550, 256388, 654003, 1689973, 4355457, 11305165, 29347706, 76527893

Offset

8,3

Comments

a(n) is the number of free ouroboros polyhexes, i.e., polyhexes in which every cell has exactly two neighbors. There are also 1 ouroboros polyhex with 3 cells and 1 with 6 cells, which are not included here because in Cyvin et al. (1991) it is required that "the inner perimeter should have a length of at least 10 edges". - Pontus von Brömssen, Jun 23 2026

Links

Table of n, a(n) for n=8..30.

Sven J. Cyvin, Jon Brunvoll, Björg N. Cyvin, John L. Bergan, and E. Brendsdal, The simplest coronoids: hollow hexagons, Struct. Chem., 2 (1991), 555-566.

Johan Nilsson, Illustrations for initial terms.

Crossrefs

Cf. A359706 (ouroboros polyominoes), A397240 (ouroboros polyiamonds).

Sequence in context: A159610 A074246 A134426 * A194608 A297870 A052973

Adjacent sequences: A122669 A122670 A122671 * A122673 A122674 A122675

Keyword

nonn,more,changed

Author

N. J. A. Sloane, Sep 23 2006

Extensions

a(15)-a(30) from Johan Nilsson, Aug 12 2015

Status

approved