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 A379626 #22 Feb 16 2025 13:54:19 %S A379626 1,1,3,9,29,91,322,1206,4600,17931,70577,279652,1110758,4424120, %T A379626 17647314,70484576,281750598,1127181327 %N A379626 Sum of the widths of the free polyominoes with n cells. %C A379626 The width here is the shorter of the two dimensions. %H A379626 <a href="/index/Pol#polyominoes">Index entries for sequences related to polyominoes</a>. %e A379626 For n = 4 the free polyominoes with four cells are also called free tetrominoes. %e A379626 The five free tetrominoes are as shown below: %e A379626 _ %e A379626 |_| _ _ _ %e A379626 |_| |_| |_|_ |_|_ _ _ %e A379626 |_| |_|_ |_|_| |_|_| |_|_| %e A379626 |_| |_|_| |_| |_| |_|_| %e A379626 . %e A379626 There is only one free tetromino of width 1 and there are four free tetrominoes of width 2, hence the sum of the widths is 1 + 2 + 2 + 2 + 2 = 9, so a(4) = 9. %Y A379626 Row sums of A379637. %Y A379626 Cf. A000105, A379623, A379625, A379627, A379629. %K A379626 nonn,more %O A379626 1,3 %A A379626 _Omar E. Pol_, Jan 16 2025 %E A379626 a(15)-a(16) from _Pontus von Brömssen_, Jan 17 2025 %E A379626 a(17)-a(18) from _John Mason_, Feb 16 2025