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 A127849 #7 Dec 31 2022 11:41:54
%S A127849 0,1,30,3875,2437500,7626953125,119201660156250,9313106536865234375,
%T A127849 3637969493865966796875000,7105423719622194766998291015625,
%U A127849 69388931933644926175475120544433593750
%N A127849 a(n) = 5^C(n,2)*(5^n-1)/4.
%C A127849 -a(n) is the Hankel transform of A127848, the series reversion of x/(1+6*x+5*x^2). Note that (5^n-1)/4 has g.f. x/(1-6*x+5*x^2).
%C A127849 (-1)^n*a(n) is the Hankel transform of the series reversion of x*(1+4*x)/(1+2*x-3*x^2). - _Paul Barry_, Jul 12 2008
%t A127849 Table[5^Binomial[n,2] (5^n-1)/4,{n,0,20}] (* _Harvey P. Dale_, Dec 31 2022 *)
%Y A127849 Cf. A127848.
%K A127849 easy,nonn
%O A127849 0,3
%A A127849 _Paul Barry_, Feb 02 2007