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 A360421 #16 Feb 18 2023 21:15:24 %S A360421 0,0,0,0,1,2,7,20,49,109,216,414,723,1228,1958,3065,4580,6734 %N A360421 a(n) = the number of X-frame polyominoes with n cells, reduced for symmetry. %C A360421 An X-frame polyomino has a perimeter that forms a self-avoiding polygon such that as you traverse the perimeter counterclockwise you encounter turns in the order LLRLLRLLRLLR. %e A360421 a(5) = 1 because of: %e A360421 O %e A360421 OOO %e A360421 O %e A360421 The a(7) = 7 polyominoes are: %e A360421 O O O O O O O %e A360421 OOOOO OOOOO OOOOO OOOOO OOOO OOOO OOOO %e A360421 O O O O O O OO %e A360421 O O %Y A360421 Cf. A028247, A270060, A360419, A360420. %K A360421 nonn,more %O A360421 1,6 %A A360421 _John Mason_, Feb 06 2023