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 A231559 #21 Sep 08 2022 08:46:06
%S A231559 0,0,2,6,11,17,25,35,46,58,72,88,105,123,143,165,188,212,238,266,295,
%T A231559 325,357,391,426,462,500,540,581,623,667,713,760,808,858,910,963,1017,
%U A231559 1073,1131,1190,1250,1312,1376,1441,1507,1575,1645,1716,1788,1862,1938
%N A231559 a(n) = floor( A000326(n)/2 ).
%C A231559 First trisection of A011865.
%H A231559 Bruno Berselli, <a href="/A231559/b231559.txt">Table of n, a(n) for n = 0..1000</a>
%H A231559 <a href="/index/Rec#order_05">Index entries for linear recurrences with constant coefficients</a>, signature (3,-4,4,-3,1).
%F A231559 G.f.: x^2*(2 + x^2)/((1 + x^2)*(1 - x)^3).
%F A231559 a(n) = ( n*(3*n-1) + i^(n*(n+1)) - 1 )/4, where i=sqrt(-1).
%t A231559 Table[Floor[n (3 n - 1)/4], {n, 0, 60}]
%t A231559 CoefficientList[Series[x^2(2+x^2)/((1+x^2)(1-x)^3),{x,0,70}],x] (* or *) LinearRecurrence[{3,-4,4,-3,1},{0,0,2,6,11},70] (* _Harvey P. Dale_, Jan 28 2022 *)
%o A231559 (Magma) [Floor(n*(3*n-1)/4): n in [0..60]];
%Y A231559 Cf. pentagonal numbers: A000326.
%Y A231559 Cf. A011848 for the triangular numbers: floor(A000217/2); A007590 for the squares: floor(A000290/2); A156859 for the hexagonal numbers: floor(A000384/2).
%Y A231559 First differences: A047262.
%K A231559 nonn,easy
%O A231559 0,3
%A A231559 _Bruno Berselli_, Nov 11 2013