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 A353067 #17 Aug 20 2022 13:27:50 %S A353067 1,3,8,23,71,193,490,1221,3011,7393,18025,43847,106206,256851,619642, %T A353067 1493272,3593527,8641624,20764212,49869733,119716469,287306693, %U A353067 689316717,1653539600,3965895220,9510880953,22806565004,54685294862,131116579494,314360235644 %N A353067 a(n) is the number of free polyominoes of width 3 and size n. %C A353067 The sequence can be generated using a series of recursive formulas in a fashion similar to A352720. This latter requires around 500 formula lines, whereas a(n) requires about 62000 lines. %H A353067 John Mason, <a href="/A353067/b353067.txt">Table of n, a(n) for n = 3..400</a> %H A353067 John Mason, <a href="/A353067/a353067.txt">Java program containing formulas</a> %e A353067 a(3) = 1 because of the straight tromino. %e A353067 a(4) = 3 because of the following tetrominoes: %e A353067 O O OO %e A353067 OOO OOO OO %Y A353067 Cf. A000105, A352720 (width 2 and size n), A354850 (width 4 and size n). %K A353067 nonn %O A353067 3,2 %A A353067 _John Mason_, Apr 21 2022