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 A348848 #13 Dec 04 2021 12:43:48
%S A348848 1,2,6,19,65,224,790,2851,10424,38496,143454,538667,2035180,7729146,
%T A348848 29486904,112942373,434114384,1673766428,6471199322,25081542410,
%U A348848 97431694571,379256586232,1479022885116
%N A348848 Number of oriented polyominoes with 4n cells that have fourfold rotational symmetry centered at a vertex.
%C A348848 These are polyominoes of the regular tiling with Schläfli symbol {4,4}. For oriented polyominoes, chiral pairs are counted as two. This is one of the five sequences, along with A001168, needed to calculate the number of oriented polyominoes, A000988. It is the C90(n/4) sequence in the Shirakawa link. The calculation follows Redelmeier's method of inner rings.
%H A348848 Robert A. Russell, <a href="/A348848/b348848.txt">Table of n, a(n) for n = 1..23</a>
%H A348848 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 A348848 Toshihiro Shirakawa, <a href="https://www.gathering4gardner.org/g4g10gift/math/Shirakawa_Toshihiro-Harmonic_Magic_Square.pdf">Enumeration of Polyominoes considering the symmetry</a>, April 2012, pp. 3-4.
%e A348848 For a(1)=1, the polyomino is a 2 X 2 square. For a(2)=2, the two polyominoes are a chiral pair having a central 2 X 2 square with one cell attached to each edge of that square.
%Y A348848 Cf. A000988, A144553, A348849 (cell center).
%Y A348848 Inner rings: A324406, A324407, A324408, A324409.
%K A348848 nonn
%O A348848 1,2
%A A348848 _Robert A. Russell_, Nov 01 2021