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 A340012 #19 Feb 16 2025 08:34:01
%S A340012 1,4,3,7,3,1,9,7,1,0,4,8,0,1,9,6,7,5,7,5,6,7,8,1,1,4,5,6,0,8,6,2,6,3,
%T A340012 0,7,0,3,6,8,4,4,6,1,5,4,0,6,9,3,0,4,4,4,0,7,7,5,1,3,9,1,8,0,0,7,5,4,
%U A340012 5,6,8,3,0,7,3,8,9,0,6,4,8,6,4,0,8,3
%N A340012 Decimal expansion of Sum_{n>=3} 2/(n*(n^2 + 1)).
%C A340012 Starting from a(3) = 4, this constant represents the sum of the reciprocals of the sequence M(n) of magic constants for n X n magic squares (numbered 1 through n^2), considering n >= 3.
%C A340012 Sum_{n >= 3} 1/M(n) = 1/15 + 1/34 + 1/65 + 1/111 + 1/175 + 1/260 + ... = 1.34373197104801967... - 6/5 = 0.14373197104801967...
%H A340012 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/MagicConstant.html">Magic Constant</a>.
%F A340012 Equals Sum_{k>=3} 1/A006003(k).
%F A340012 Equals H(2 - I) + H(2 + I) - 3, where H(x) = Integral_{t=0..1} (1 - t^x)/(1 - t) dt is the function that interpolates the harmonic numbers and I is the imaginary unit. - _Stefano Spezia_, Dec 26 2020
%e A340012 0.143731971048019675756781145608626...
%t A340012 RealDigits[Re @ Sum[2/(n*(n^2 + 1)), {n, 3, Infinity}], 10, 100][[1]] (* _Amiram Eldar_, Dec 26 2020 *)
%o A340012 (PARI) sumpos(n=3, 2/(n*(n^2 + 1))) \\ _Michel Marcus_, Dec 26 2020
%Y A340012 Cf. A006003, A340012.
%K A340012 cons,nonn
%O A340012 0,2
%A A340012 _Marco Ripà_, Dec 26 2020