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 A293499 #22 May 18 2019 20:39:35
%S A293499 1,2,5,14,42,132,428,1415,4730,15901,53593,180809,610157,2058962,
%T A293499 6947145,23437854,79067006,266717300,899693960,3034814143,10236853534,
%U A293499 34530252629,116475001757,392885252033
%N A293499 Number of unlabeled hereditary semiorders on n points.
%C A293499 A semiorder (poset avoiding the subposets 2+2 and 1+3, or an interval order having a representation in which all intervals have the same length) is hereditary if every initial subsequence of the ascent sequence associated to the semiorder by the bijection of Bousquet-Mélou et al. corresponds to a semiorder.
%D A293499 M. T. Keller and S. J. Young, Hereditary semiorders and enumeration of semiorders by dimension. Preprint (2017).
%H A293499 M. Bousquet-Mélou, A. Claesson, M. Dukes, and S. Kitaev, <a href="https://doi.org/10.1016/j.jcta.2009.12.007">(2+2)-free posets, ascent sequences and pattern avoiding permutations</a>, J. Combin. Theory Ser. A 117, 7 (2010), 884-909.
%H A293499 Mitchel T. Keller, Stephen J. Young, <a href="https://arxiv.org/abs/1801.00501">Hereditary Semiorders and Enumeration of Semiorders by Dimension</a>, arXiv:1801.00501 [math.CO], (2018)
%H A293499 <a href="/index/Rec#order_05">Index entries for linear recurrences with constant coefficients</a>, signature (8,-23,29,-14,1).
%F A293499 G.f.: -x*(1 - 6*x + 12*x^2 - 9*x^3 + x^4) / ( (x-1)*(x^4 - 13*x^3 + 16*x^2 - 7*x + 1) ).
%t A293499 CoefficientList[ Series[(-1 +6x -12x^2 +9x^3 -x^4)/(-1 +8x -23x^2 +29x^3 -14x^4 +x^5), {x, 0, 26}], x] (* or *)
%t A293499 LinearRecurrence[{8, -23, 29, -14, 1}, {1, 2, 5, 14, 42}, 27] (* _Robert G. Wilson v_, Jan 07 2018 *)
%Y A293499 Cf. A022493.
%K A293499 nonn
%O A293499 1,2
%A A293499 _Mitchel T. Keller_, Oct 10 2017