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%I #22 Jan 23 2021 22:31:38
%S 9,2,3,7,5,5,1,6,8,1,0,0,5,3,5,3,0,8,7,1,1,9,8,6,0,2,9,7,9,3,0,2,4,3,
%T 5,3,9,6,6,2,7,9,0,0,6,4,1,2,5,1,7,2,5,1,7,0,7,7,1,2,8,4,8,3,2,5,1,5,
%U 0,9,8,3,3,2,5,3,0,9,7,5,7,2,8,7,2,8,3,2,2,1,8,0,1,1,2,2,5,9,9,9,6,2,6,3,5
%N Decimal expansion of Cl_2(Pi/5), where Cl_2 is the Clausen function of order 2.
%H For links see A261024.
%F A = Cl_2(Pi/5).
%F B = Cl_2(2*Pi/5).
%F C = Cl_2(3*Pi/5).
%F D = Cl_2(4*Pi/5).
%F 4*(A^2 + C^2) = 5*(B^2 + D^2).
%F B = 2*A - 2*D.
%F D = 2*B - 2*C.
%F 2*C = 4*A - 5*D.
%F B = -D + sqrt(A*(2*C+D)+D^2).
%F B^2 + D^2 = 4*Pi^4/(325*A340628^2).
%F B^2 + D^2 = (13/1125)*A340629^2*Pi^4.
%F Equals Pi*(2*log(G(9/10) / G(11/10)) + log(Pi*(1+sqrt(5)))/5), where G is the Barnes G-function. - _Vaclav Kotesovec_, Jan 23 2021
%e 0.9237551681005353087119860297930...
%t Cl2[x_] := (I/2)*(PolyLog[2, Exp[-I*x]] - PolyLog[2, Exp[I*x]]); RealDigits[Re[Cl2[Pi/5]], 10, 105] // First
%t N[Pi*(ArcCsch[2] + Log[2*Pi*BarnesG[9/10]^10 / BarnesG[11/10]^10])/5, 120] (* _Vaclav Kotesovec_, Jan 23 2021 *)
%Y Cf. A006752 (Cl_2(Pi/2) = Catalan's constant), A143298 (Cl_2(Pi/3) = Gieseking's constant), A261025 (Cl_2(Pi/4)), A261026 (Cl_2(3*Pi/4)), A261027 (Cl_2(Pi/6)), A261028 (Cl_2(5*Pi/6)), A340628, A340629.
%K nonn,cons
%O 1,1
%A _Artur Jasinski_, Jan 23 2021
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