A034184 Not necessarily symmetric n X 3 crossword puzzle grids.

1, 15, 111, 649, 3495, 18189, 93231, 474479, 2406621, 12187137, 61668609, 311938233, 1577602849, 7977940187, 40342860995, 204001993697, 1031568839407, 5216271035257, 26376744398811, 133377264694375

Offset: 1

Author

Erich Friedman

Keywords

• nonn

Links

• Andrew Howroyd, Table of n, a(n) for n = 1..200
• Louis Marin, Counting Polyominoes in a Rectangle b X h, arXiv:2406.16413 [cs.DM], 2024. See p. 145.

Crossrefs

Row 3 of A292357.

Column sums of A059679.

Cf. A001333, A034182, A034184, A034185, A034186, A034187.

Formula

Appears to obey a 9-term linear recurrence. - Ralf Stephan, May 05 2004

Empirical g.f.: -x*(x^8-x^7-2*x^6-9*x^5+14*x^4-11*x^3-6*x^2+5*x+1) / ((x-1)*(x^2+2*x-1)*(x^6-7*x^5+x^4+6*x^3-11*x^2+7*x-1)). - Colin Barker, Jun 09 2013