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 A221291 #5 Jul 23 2025 01:49:17 %S A221291 1,12,92,673,5912,48298,396846,3240527,26241353,211785583,1703772887, %T A221291 13679840407,109689414233,878752161168,7035829547262,56311795445487 %N A221291 Equals two maps: number of 3Xn binary arrays indicating the locations of corresponding elements equal to exactly two of their horizontal and antidiagonal neighbors in a random 0..3 3Xn array. %C A221291 Row 3 of A221290 %e A221291 Some solutions for n=3 %e A221291 ..0..1..1....0..0..0....0..0..0....0..1..0....0..0..1....0..1..0....0..0..0 %e A221291 ..0..1..0....1..1..0....0..1..0....0..0..1....0..1..0....0..0..0....0..0..1 %e A221291 ..0..0..0....0..1..0....0..1..0....0..1..0....0..0..0....0..1..0....1..1..0 %K A221291 nonn %O A221291 1,2 %A A221291 _R. H. Hardin_ Jan 09 2013