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%I #25 Feb 11 2021 05:49:28
%S 1,8,9,5,1,1,7,8,1,6,3,5,5,9,3,6,7,5,5,4,6,6,5,2,0,9,3,4,3,3,1,6,3,4,
%T 2,6,9,0,1,7,0,6,0,5,8,1,7,3,2,7,0,7,5,9,1,6,4,6,2,2,8,4,3,1,8,8,2,5,
%U 1,3,8,3,4,5,3,3,8,0,4,1,5,3,5,4,8,9,0,0,7,1,0,1,2,6,1,3,8,9,5,6,9,7,1,8,1
%N Decimal expansion of second exponential integral at 1, ExpIntegralEi[1].
%H Robert Price, <a href="/A091725/b091725.txt">Table of n, a(n) for n = 1..10000</a>
%H Zbigniew Gołębiewski, Mateusz Klimczak, <a href="https://doi.org/10.1137/1.9781611975505.5">Protection Number of Recursive Trees</a>, 2019 Proceedings of the Sixteenth Workshop on Analytic Algorithmics and Combinatorics (ANALCO).
%H Eric Weisstein's World of Mathematics, <a href="http://mathworld.wolfram.com/ExponentialIntegral.html">Exponential Integral</a>
%H Eric Weisstein's World of Mathematics, <a href="http://mathworld.wolfram.com/FactorialSums.html">Factorial Sums</a>
%H Eric Weisstein's World of Mathematics, <a href="http://mathworld.wolfram.com/LogarithmicIntegral.html">Logarithmic Integral</a>
%F Equals li(e), where li(x) is the logarithmic integral, and gamma + Sum_{n>=1} 1/(n*n!) = A001620 + A229837. - _Amiram Eldar_, Mar 05 2019
%e 1.89511781635593675546652...
%t RealDigits[ExpIntegralEi[1], 10, 105][[1]] (* _Robert G. Wilson v_, Oct 08 2004 *)
%o (PARI) real(-eint1(-1)) \\ _Charles R Greathouse IV_, Apr 23 2013
%Y Cf. A001563, A001620, A229837.
%K nonn,cons
%O 1,2
%A _Eric W. Weisstein_, Feb 01 2004
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