OFFSET
1,1
REFERENCES
W. G. Bickley and J. C. P. Miller, Numerical differentiation near the limits of a difference table, Phil. Mag., 33 (1942), 1-12 (plus tables).
A. Fletcher, J. C. P. Miller, L. Rosenhead, and L. J. Comrie, An Index of Mathematical Tables. Vols. 1 and 2, 2nd ed., Blackwell, Oxford and Addison-Wesley, Reading, MA, 1962, Vol. 1, p. 267.
J. W. L. Glaisher, Phil. Trans. Royal Society, 160 (1870), 367-388.
N. J. A. Sloane, A Handbook of Integer Sequences, Academic Press, 1973 (includes this sequence).
N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
LINKS
Vincenzo Librandi, Table of n, a(n) for n = 1..1000
FORMULA
Ei(x) = Integral_{-oo..x} (e^t/t) dt.
a(n) ~ exp(n)/n. - Vaclav Kotesovec, May 23 2014
MATHEMATICA
a[n_] := Round[ExpIntegralEi[n]]; Table[a[n], {n, 1, 30}] (* Jean-François Alcover, Dec 19 2011 *)
PROG
(PARI) a(n)=round(-eint1(-n))
CROSSREFS
KEYWORD
nonn,nice
AUTHOR
STATUS
approved