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 A259090 #34 Mar 21 2024 21:00:51
%S A259090 1,1,2,4,7,15,24,53,89,194,323,722,1197,2706,4465,10229,16814,38900,
%T A259090 63721,148808,243006,571937,931305,2207561,3584959,8551309,13851410,
%U A259090 33229843,53694326,129485500,208738348,505796115,813532119,1980023840,3177764415
%N A259090 Number of symmetrical n-celled polyominoes.
%H A259090 John Mason, <a href="/A259090/b259090.txt">Table of n, a(n) for n = 1..50</a> (This version corrects erroneous values from a(44) onwards in previous version)
%H A259090 T. R. Parkin, L. J. Lander, and D. R. Parkin, <a href="/A000104/a000104.pdf">Polyomino Enumeration Results</a>, presented at SIAM Fall Meeting, 1967, and accompanying letter from T. J. Lander (annotated scanned copy). See page 21. The 15th entry is not included because their value for A000105(15) was incorrect.
%F A259090 a(n) = A000105(n) - A006749(n).
%t A259090 A000105 = Cases[Import["https://oeis.org/A000105/b000105.txt", "Table"], {_, _}][[All, 2]];
%t A259090 A006749 = Cases[Import["https://oeis.org/A006749/b006749.txt", "Table"], {_, _}][[All, 2]];
%t A259090 a[n_] := A000105[[n + 1]] - A006749[[n]];
%t A259090 Array[a, 28] (* _Jean-François Alcover_, Aug 27 2019 *)
%Y A259090 Cf. A000105, A006749.
%K A259090 nonn
%O A259090 1,3
%A A259090 _N. J. A. Sloane_, Jun 18 2015
%E A259090 a(5)-a(6) corrected and a(15)-a(28) from _Andrew Howroyd_, Dec 04 2018
%E A259090 a(29) and beyond from _John Mason_, Dec 22 2021