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 A293007 #15 Sep 29 2017 16:22:47
%S A293007 0,0,2,4,12,32,88,240,656,1792,4896,13376,36544,99840,272768,745216,
%T A293007 2035968,5562368,15196672,41518080,113429504,309895168,846649344,
%U A293007 2313089024,6319476736,17265131520,47169216512,128868696064,352075825152,961889042432
%N A293007 Expansion of 2*x^2 / (1-2*x-2*x^2).
%C A293007 Number of associative, quasitrivial, and order-preserving binary operations on the n-element set {1,...,n} that have neutral and annihilator elements.
%H A293007 Colin Barker, <a href="/A293007/b293007.txt">Table of n, a(n) for n = 0..1000</a>
%H A293007 M. Couceiro, J. Devillet, and J.-L. Marichal, <a href="http://arxiv.org/abs/1709.09162">Quasitrivial semigroups: characterizations and enumerations</a>, arXiv:1709.09162 [math.RA] (2017).
%H A293007 <a href="/index/Rec#order_02">Index entries for linear recurrences with constant coefficients</a>, signature (2,2).
%F A293007 a(n) = 2*A002605(n-1), a(0) = 0.
%F A293007 a(n) = A028860(n+1), a(0) = 0.
%F A293007 From _Colin Barker_, Sep 28 2017: (Start)
%F A293007 a(n) = ((1-sqrt(3))^n*(1+sqrt(3)) + (-1+sqrt(3))*(1+sqrt(3))^n) / (2*sqrt(3)) for n>0.
%F A293007 a(n) = 2*a(n-1) + 2*a(n-2) for n>2.
%F A293007 (End)
%o A293007 (PARI) concat(vector(2), Vec(2*x^2 / (1-2*x-2*x^2) + O(x^50))) \\ _Colin Barker_, Sep 28 2017
%Y A293007 Cf. A002605, A293005, A293006.
%Y A293007 Essentially the same as A028860 and A152035.
%K A293007 nonn,easy
%O A293007 0,3
%A A293007 _J. Devillet_, Sep 28 2017