A034185 Number of symmetric n X 3 crossword puzzle grids.

1, 3, 11, 21, 61, 111, 309, 565, 1563, 2859, 7907, 14459, 39991, 73117, 202227, 369729, 1022587, 1869581, 5170829, 9453757, 26146909, 47804041, 132214959, 241726811, 668560673, 1222320359, 3380656559, 6180808253, 17094691953, 31253992007, 86441342981

Offset: 1

Author

Erich Friedman

Keywords

• nonn

Comments

Rules: all white squares are edge connected; at least 1 white square on every edge of grid; symmetric ones have 180-degree rotational symmetry.

Links

• Sean A. Irvine, Java program (github)

Crossrefs

Cf. A001333, A034184, A034186, A034187.

Extensions

a(23)-a(31) from Sean A. Irvine, Aug 07 2020