This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A366443 #22 Apr 14 2025 11:02:07 %S A366443 0,0,0,1,0,1,1,5,5,23,46,187,552,2145,7818 %N A366443 Number of free polyominoes of site-perimeter n. %C A366443 This sequence counts free connected (via common edges) polyominoes with given site-perimeter. The site-perimeter of a polyomino is the number of cells that are adjacent to it (via common edges). This sequence allows holes of any kind. %H A366443 John Mason, <a href="/A366443/a366443_2.pdf">Examples</a> %e A366443 a(4) = a(6) = a(7) = 1 as the monomino, domino and L-shaped tromino are the only polyominoes with site perimeter 4, 6 and 7 respectively. %e A366443 a(5) = 0 as no polyomino has a site-perimeter of 5. %e A366443 a(8) = 5 as the straight tromino, square tetromino, T-tetromino, S-tetromino and cross pentomino are the only polyominoes with site perimeter 8. See link "Examples". %Y A366443 Cf. A000105 (free polyominoes), A001971 (the maximum size of a polyomino with site-perimeter n is given by A001971(n-2)), A057730 (perimeter instead of site-perimeter), A216820 (fixed version of current sequence). %Y A366443 Column sums of A338211 (without the column for 0-celled polyominoes). %K A366443 nonn,hard,more %O A366443 1,8 %A A366443 _John Mason_ from an idea of _Allan C. Wechsler_, Oct 10 2023 %E A366443 a(15) corrected by _Sean A. Irvine_, Apr 13 2025