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 A215141 #11 Apr 20 2018 11:49:12
%S A215141 9,0,4,2,2,6,1,3,8,2,2,8,4,3,4,5,2,1,1,4,6,2,7,4,9,6,5,6,2,5,6,5,6,2,
%T A215141 7,3,2,8,7,5,1,8,2,8,3,4,9,0,3,8,1,7,6,7,3,5,9,9,7,9,1,0,9,8,7,5,8,3,
%U A215141 2,8,2,4,3,4,7,8,8,1,4,3,0,3,6,0,5,6,8
%N A215141 Decimal expansion of Sum_{n>=1} (-1)^(n+1) / sigma(n)^n.
%H A215141 G. C. Greubel, <a href="/A215141/b215141.txt">Table of n, a(n) for n = 0..5000</a>
%e A215141 .90422613822843452114627...
%t A215141 RealDigits[N[Sum[((-1)^(n+1))/DivisorSigma[1,n]^n,{n,1,105}],105]][[1]]
%o A215141 (PARI) suminf(n=1, (-1)^(n+1)/sigma(n)^n) \\ _Michel Marcus_, Apr 20 2018
%Y A215141 Cf. A000203, A215140.
%K A215141 nonn,cons
%O A215141 0,1
%A A215141 _Michel Lagneau_, Aug 04 2012