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 A326301 #14 Sep 24 2024 14:41:27 %S A326301 1,2,3,8,19,54,159,511,1676,5685,19512 %N A326301 The number of rectangular free polyominoes with n cells where no 4 cells meet at a point. %C A326301 Obtained from the polyominoes of A056780 by discarding the polyominoes that contain 2 X 2 subblocks of 4 cells. One may call this the Tatami version of the free rectangular polyominoes. %C A326301 This might also be considered a well-defined (or corrected?) version of A002369. %e A326301 From the A056780(4) = 9 polyominoes the one with the 2 X 2 bounding box is not counted, giving a(4)=8. %e A326301 From the A056780(5)=21 polyominoes, 2 are not counted: %e A326301 L %e A326301 LL and LL %e A326301 LL LLL %Y A326301 Cf. A056780, A002369. %K A326301 nonn,more %O A326301 1,2 %A A326301 _R. J. Mathar_, Oct 17 2019