%I #33 Nov 21 2022 09:38:47
%S 2,2,0,3,8,5,6,5,9,6,4,3,7,8,5,9,7,8,7,8,7,2,8,2,8,3,1,6,4,8,0,0,8,9,
%T 6,6,2,5,6,7,1,7,3,1,9,3,7,8,7,8,5,8,6,3,4,1,7,0,4,9,5,5,4,4,8,7,1,6,
%U 6,8,8,6,8,1,1,8,5,2,6,9,5,4,9,7,5,7,2,6,6,0,4,1,9,0,1,3,9,5,6
%N Decimal expansion of Product_{p prime >= 2} (1 + p/((p-1)^2*(p+1))).
%C Decimal expansion of totient constant. - _Eric W. Weisstein_, Apr 20 2006
%H Steven R. Finch, <a href="https://doi.org/10.1017/9781316997741">Mathematical Constants II</a>, Encyclopedia of Mathematics and Its Applications, Cambridge University Press, Cambridge, 2018, p. 86.
%H G. Niklasch, <a href="/A001692/a001692.html">Some number theoretical constants: 1000-digit values</a>. [Cached copy]
%H Eric Weisstein's World of Mathematics, <a href="http://mathworld.wolfram.com/TotientSummatoryFunction.html">Totient Summatory Function</a>.
%F Equals Pi^2 * A065483 / 6.
%F Also defined as: Sum_{m>=1} 1/(m*A000010(m)). See Weisstein link.
%F Equals 5 * Sum_{m>=1} (-1)^(m+1)/(m*A000010(m)). - _Amiram Eldar_, Nov 21 2022
%e 2.203856596437859787872828316480...
%t $MaxExtraPrecision = 500; digits = 99; terms = 500; P[n_] := PrimeZetaP[n];
%t LR = Join[{0, 0, 0}, LinearRecurrence[{2, -1, -1, 1}, {3, 4, 5, 3}, terms + 10]]; r[n_Integer] := LR[[n]]; (Pi^2/6)*Exp[NSum[r[n]*P[n - 1]/(n - 1), {n, 3, terms}, NSumTerms -> terms, WorkingPrecision -> digits + 10] ] // RealDigits[#, 10, digits]& // First (* _Jean-François Alcover_, Apr 18 2016 *)
%o (PARI) prodeulerrat(1 + p/((p-1)^2*(p+1))) \\ _Hugo Pfoertner_, Jun 02 2020
%Y Cf. A000010, A002618, A065483, A077387.
%K cons,nonn
%O 1,1
%A _N. J. A. Sloane_, Nov 19 2001, Aug 09 2010