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 A118347 #7 Mar 17 2021 22:51:11
%S A118347 1,2,11,70,487,3592,27600,218566,1771367,14621410,122495659,
%T A118347 1038934480,8903129300,76970244560,670507216168,5879770542870,
%U A118347 51861650744071,459804626981158,4095433894576785,36628711884398086,328824295880947471
%N A118347 Semi-diagonal (one row below central terms) of pendular triangle A118345 and equal to the self-convolution of the central terms (A118346).
%H A118347 G. C. Greubel, <a href="/A118347/b118347.txt">Table of n, a(n) for n = 0..500</a>
%F A118347 G.f.: ( series_inverse( x*(1-2*x +sqrt((1-2*x)*(1-6*x)))/(2*(1-2*x)) )/x )^2.
%t A118347 CoefficientList[(InverseSeries[Series[x*(1-2*x +Sqrt[(1-2*x)*(1-6*x)])/(2*(1-2*x)), {x, 0, 30}]]/x)^2, x] (* _G. C. Greubel_, Mar 17 2021 *)
%o A118347 (PARI) {a(n) = polcoeff( (serreverse(x*(1-2*x+sqrt((1-2*x)*(1-6*x)+x*O(x^n)))/(2*(1-2*x)))/x)^2,n)}
%o A118347 (Sage)
%o A118347 def A118347_list(prec):
%o A118347 P.<x> = PowerSeriesRing(ZZ, prec)
%o A118347 return P( (( x*(1-2*x +sqrt((1-2*x)*(1-6*x)))/(2*(1-2*x)) ).reverse()/x)^2 ).list()
%o A118347 A118347_list(31) # _G. C. Greubel_, Mar 17 2021
%o A118347 (Magma)
%o A118347 R<x>:=PowerSeriesRing(Rationals(), 30);
%o A118347 Coefficients(R!( (Reversion( x*(1-2*x +Sqrt((1-2*x)*(1-6*x)))/(2*(1-2*x)) )/x)^2 )); // _G. C. Greubel_, Mar 17 2021
%Y A118347 Cf. A118345, A118346, A118348, A118349.
%K A118347 nonn
%O A118347 0,2
%A A118347 _Paul D. Hanna_, Apr 26 2006