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 A342537 #14 Jan 04 2023 11:55:00 %S A342537 1,1,2,5,12,35,108,368,1283,4639,17003,63222,236819,893515,3387953, %T A342537 12904855,49335965,189243115 %N A342537 Number of free polyominoes with n cells having simply-connected interiors. %C A342537 The polyominoes referred to in the definition are those having a single traceable perimeter. The definition therefore includes the heptomino with a hole, as in this case the border, although not self-avoiding (it meets itself at a corner) is still traceable; it excludes the square octomino with a hole, whose border consists of two distinct polygons. Thus starting from n=8, A000104(n) < a(n) < A000105(n). %Y A342537 Cf. A000104 (free polyominoes without holes), A000105 (free polyominoes). %K A342537 nonn,more %O A342537 1,3 %A A342537 _John Mason_, _Allan C. Wechsler_, Mar 15 2021 %E A342537 a(17)-a(18) from _John Mason_, Jan 04 2023