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 A368657 #23 Jan 31 2024 08:06:46 %S A368657 0,0,1,13,167,2685,50391,1188935,41749885,2645126227,341643017303, %T A368657 82472721488013,31312529515504513,17381378412860375479, %U A368657 14419291783372365769995,18997663191047558313462721 %N A368657 Number of cycles in an n X n grid where the cycle cannot touch itself orthogonally or diagonally and must contain at least one inside point. %C A368657 For n > 1, n < 5, this shares the sequence with n-1 in A140517. Cycles are not reduced by symmetry (rotation, translation or mirroring). The grid can only have one cycle. %H A368657 Cracking The Cryptic, <a href="https://www.youtube.com/watch?v=hgx0Rso7JNs">The Trouble With Circular Reasoning Is.... (!)</a>, YouTube. %H A368657 Jimmy Mårdell, <a href="https://github.com/Yarin78/puzzles/blob/master/src/puzzles/grid_loop.py">Python code used for generation of the sequence</a>. %e A368657 For n = 4, there are 13 valid cycles: %e A368657 . %e A368657 1 2 3 4 %e A368657 ###. .### .... .... %e A368657 #.#. .#.# .### ###. %e A368657 ###. .### .#.# #.#. %e A368657 .... .... .### ###. %e A368657 . %e A368657 5 6 7 8 %e A368657 #### .... ###. .### %e A368657 #..# #### #.#. .#.# %e A368657 #### #..# #.#. .#.# %e A368657 .... #### ###. .### %e A368657 . %e A368657 9 10 11 12 %e A368657 .### ###. #### #### %e A368657 ##.# #.## #..# #..# %e A368657 #..# #..# #.## ##.# %e A368657 #### #### ###. .### %e A368657 . %e A368657 13 %e A368657 #### %e A368657 #..# %e A368657 #..# %e A368657 #### %Y A368657 Cf. A140517, A297664. %K A368657 nonn,more %O A368657 1,4 %A A368657 _Niklas Gustavsson_, Jan 02 2024