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 A249273 #10 Jun 03 2017 15:31:21
%S A249273 2,5,3,5,0,5,4,1,8,0,3,6,0,4,3,8,8,3,0,1,6,5,5,3,0,0,0,7,1,8,5,9,0,8,
%T A249273 3,5,0,8,6,1,1,7,8,0,1,3,8,5,3,7,0,1,6,4,5,3,7,7,5,1,2,6,4,9,4,3,6,4,
%U A249273 1,4,7,5,3,8,2,9,6,7,8,5,4,7,0,1,7,0,3,3,6,6,5,1,7,9,1,0,9,0,3,4,2,4,5
%N A249273 Decimal expansion of a constant associated with the set of all complex nonprincipal Dirichlet characters.
%H A249273 Steven R. Finch, <a href="/A232927/a232927.pdf">Average least nonresidues</a>, December 4, 2013. [Cached copy, with permission of the author]
%H A249273 G. Martin and P. Pollack, <a href="http://dx.doi.org/10.1112/jlms/jds036">The average least character non-residue and further variations on a theme of Erdős</a>, J. London Math. Soc. 87 (2013) 22-42.
%F A249273 sum_{k >= 1} p_k^2/((p_1 + 1)(p_2 + 1)...(p_k + 1)), where p_k is the k-th prime number.
%e A249273 2.5350541803604388301655300071859083508611780138537...
%t A249273 digits = 103; Clear[s]; s[m_] := s[m] = Sum[Prime[k]^2/Product[Prime[j] + 1, {j, 1, k}] , {k, 1, m}] // N[#, digits + 100]&; s[10]; s[m = 20]; While[RealDigits[s[m]] != RealDigits[s[m/2]], Print[m, " ", N[s[m]]]; m = 2*m]; RealDigits[s[m], 10, digits] // First
%Y A249273 Cf. A232929, A232930, A232931, A232932.
%K A249273 nonn,cons
%O A249273 1,1
%A A249273 _Jean-François Alcover_, Oct 24 2014