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 A020933 #15 Jul 08 2025 07:57:17
%S A020933 1,-42,798,-9044,67830,-352716,1293292,-3325608,5819814,-6466460,
%T A020933 3879876,-705432,-117572,-54264,-38760,-36176,-40698,-52668,-76076,
%U A020933 -120120,-204204,-369512,-705432,-1410864,-2939300,-6348888,-14162904,-32522224,-76659528,-185040240
%N A020933 Expansion of (1-4*x)^(21/2).
%F A020933 D-finite with recurrence: n*a(n) +2*(-2*n+23)*a(n-1)=0. - _R. J. Mathar_, Jan 17 2020
%F A020933 From _Amiram Eldar_, Mar 25 2022: (Start)
%F A020933 a(n) = (-4)^n*binomial(21/2, n).
%F A020933 Sum_{n>=0} 1/a(n) = 406240/415701 - 46*Pi/(3^13*sqrt(3)).
%F A020933 Sum_{n>=0} (-1)^n/a(n) = 728323714975904/710426513671875 - 92*log(phi)/(5^12*sqrt(5)), where phi is the golden ratio (A001622). (End)
%t A020933 CoefficientList[Series[Surd[(1-4x)^21,2],{x,0,30}],x] (* _Harvey P. Dale_, Feb 25 2020 *)
%Y A020933 Cf. A001622, A002420, A002421, A002422, A002423, A002424, A020923, A020925, A020927, A020929, A020931.
%K A020933 sign
%O A020933 0,2
%A A020933 _N. J. A. Sloane_