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 A381820 #10 Mar 08 2025 09:40:02
%S A381820 1,2,20,281,4599,82113,1550993,30473930,616463800,12753523628,
%T A381820 268586285058,5738804673016,124098812744140,2710824280371114,
%U A381820 59728504549831296,1325862161472193292,29623682752417138511,665679666998856945540,15034747192791290846435
%N A381820 Expansion of ( (1/x) * Series_Reversion( x * ((1-x) / C(x))^4 ) )^(1/4), where C(x) is the g.f. of A000108.
%F A381820 G.f. A(x) satisfies A(x) = C(x*A(x)^4) / (1 - x*A(x)^4).
%F A381820 a(n) = Sum_{k=0..n} binomial(4*n+2*k+1,k) * binomial(5*n-k,n-k)/(4*n+2*k+1).
%o A381820 (PARI) my(N=20, x='x+O('x^N)); Vec((serreverse(x*((1-x)*2*x/(1-sqrt(1-4*x)))^4)/x)^(1/4))
%Y A381820 Cf. A381817, A381818, A381819.
%Y A381820 Cf. A000108, A381774.
%K A381820 nonn
%O A381820 0,2
%A A381820 _Seiichi Manyama_, Mar 07 2025