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 A359520 #8 Jan 05 2023 11:04:38 %S A359520 0,0,0,0,0,0,1,5,35,179,909,4285,19702,88040,386826,1674852,7174436, %T A359520 30462009 %N A359520 Number of free holey polyominoes of n cells with simply-connected interiors. %F A359520 a(n) = A342537(n) - A000104(n) (simply connected polyominoes minus unholey polyominoes). %e A359520 a(7) = 1 because of the holey heptomino. %Y A359520 Cf. A000104, A000105, A001419, A342537, A359519. %K A359520 nonn,more %O A359520 1,8 %A A359520 _John Mason_, Jan 04 2023