OFFSET
0,1
COMMENTS
li(x) is the logarithmic integral function, extended to the whole complex plane. The corresponding imaginary part is in A257818.
LINKS
Stanislav Sykora, Table of n, a(n) for n = 0..2000
Eric Weisstein's World of Mathematics, Logarithmic Integral
Wikipedia, Logarithmic integral function
FORMULA
Equals gamma + log(Pi/2) + Sum_{k>=1}((-1)^k*(Pi/2)^(2*k)/(2*k)!/(2*k)).
Equals Ci(Pi/2), the maximum value of the cosine integral along the real axis. - Stanislav Sykora, Nov 12 2016
EXAMPLE
0.47200065143956865077760610761412783650733054301836188186838371...
MAPLE
evalf(Re(Li(I)), 120); # Vaclav Kotesovec, May 10 2015
MATHEMATICA
RealDigits[Re[LogIntegral[I]], 10, 120][[1]] (* Vaclav Kotesovec, May 10 2015 *)
PROG
(PARI) li(z) = {my(c=z+0.0*I); \\ If z is real, convert it to complex
if(imag(c)<0, return(-Pi*I-eint1(-log(c))),
return(+Pi*I-eint1(-log(c)))); }
a=real(li(I))
CROSSREFS
KEYWORD
nonn,cons
AUTHOR
Stanislav Sykora, May 10 2015
STATUS
approved