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 A380575 #19 Mar 02 2025 23:19:59
%S A380575 2,3,8,24,71,236,835,3182,12302,48834,195035,785280,3167322,12808531,
%T A380575 51834644,209965222,850817523,3449091525
%N A380575 One half of the sum of the perimeters of the free polyominoes with n cells.
%C A380575 A380287 is the main entry for this question.
%C A380575 One half of the perimeters of any holes are included here.
%H A380575 <a href="/index/Pol#polyominoes">Index entries for sequences related to polyominoes</a>.
%F A380575 a(n) = A380287(n)/2.
%F A380575 a(n) = Sum_{k=2..n+1} k*A342243(n,k). - _Andrew Howroyd_, Feb 28 2025
%Y A380575 Cf. A000105, A342243, A380287.
%K A380575 nonn,more
%O A380575 1,1
%A A380575 _Omar E. Pol_, Feb 12 2025