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 A350243 #11 Dec 25 2021 13:51:44
%S A350243 1,1,2,5,9,19,39,82,171,368,773,1678,3559,7776,16601,36470,78295,
%T A350243 172720,372440,824512,1784463,3961869,8601227,19143685,41671452,
%U A350243 92944943,202787164,453138925,990656774,2217280465,4856097782,10884558781,23876327783,53585821550,117713147451
%N A350243 Number of achiral hexagonal polyominoes with 3n cells and threefold rotational symmetry centered at a vertex.
%C A350243 These are polyominoes of the regular tiling with Schläfli symbol {6,3}. Each has a symmetry group of order 6. This sequence along with five others and A001207 can be used to determine A006535, the number of oriented polyominoes of the {6,3} regular tiling.
%C A350243 The sequence is calculated by using Redelmeier's method to generate fixed polyominoes, which are then mapped to one or two of the symmetric polyominoes as shown in the attachment.
%H A350243 D. H. Redelmeier, <a href="http://dx.doi.org/10.1016/0012-365X(81)90237-5">Counting polyominoes: yet another attack</a>, Discrete Math., 36 (1981), 191-203.
%H A350243 Robert A. Russell, <a href="/A350243/a350243.pdf">Mapping fixed polyominoes</a>
%e A350243 For a(1)=1, a(2)=1, and a(3)=2, the polyominoes are:
%e A350243 X X X X X X
%e A350243 X X X X X X X
%e A350243 X X X X X X X X X
%e A350243 X X X X X
%Y A350243 Cf. A001207, A006535.
%K A350243 nonn
%O A350243 1,3
%A A350243 _Robert A. Russell_, Dec 21 2021