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 A206531 #14 Dec 22 2022 02:11:52
%S A206531 0,2,58,3250,292498,38609738,7026972314,1686473355362,516060846740770,
%T A206531 196103121761492602,90599642253809582122,50011002524102889331346,
%U A206531 32507151640666878065374898,24575406640344159817423422890
%N A206531 a(n) = (2*(n+1)*(2*n+1)-1)*a(n-1) + 2*n*(2*n-1)*a(n-2), a(0)=0, a(1)=2.
%C A206531 The numerators of the fractions limiting to the value of A206533.
%D A206531 E. W. Cheney, Introduction to Approximation Theory, McGraw-Hill, Inc., 1966.
%H A206531 G. C. Greubel, <a href="/A206531/b206531.txt">Table of n, a(n) for n = 0..220</a>
%F A206531 a(n) = A082108(n)*a(n-1) + A002939(n)*a(n-2), a(0) = 0, a(1) = 2.
%t A206531 RecurrenceTable[{a[n]==(2(n+1)(2n+1)-1)a[n-1]+2n(2n-1)a[n-2],a[0]==0,a[1]==2},a,{n,15}]
%o A206531 (Magma) [n le 2 select 2*(n-1) else (2*n*(2*n-1)-1)*Self(n-1) + 2*(n-1)*(2*n-3)*Self(n-2): n in [1..31]]; // _G. C. Greubel_, Dec 21 2022
%o A206531 (SageMath)
%o A206531 @CachedFunction
%o A206531 def a(n): # a = A206531
%o A206531 if (n<2): return 2*n
%o A206531 else: return (2*(n+1)*(2*n+1)-1)*a(n-1) + 2*n*(2*n-1)*a(n-2)
%o A206531 [a(n) for n in range(31)] # _G. C. Greubel_, Dec 21 2022
%Y A206531 Cf. A002939, A082108, A206532, A206533.
%K A206531 nonn
%O A206531 0,2
%A A206531 _Seiichi Kirikami_, Feb 11 2012