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Floor(Li(2n) - Li(n)).
0

%I #6 Jun 21 2017 19:29:39

%S 1,2,2,2,2,3,3,3,3,3,4,4,4,4,5,5,5,5,5,6,6,6,6,6,7,7,7,7,7,8,8,8,8,8,

%T 9,9,9,9,9,10,10,10,10,10,10,11,11,11,11,11,12,12,12,12,12,12,13,13,

%U 13,13,13,13,14,14,14,14,14,14,15,15,15,15,15,15,16,16,16,16,16,16,17,17,17

%N Floor(Li(2n) - Li(n)).

%F Li(x) = Int(t=2, x, dt/log(t)) = Logarithmic Integral.

%t Table[Floor[LogIntegral[2n]-LogIntegral[n]],{n,2,90}] (* _Harvey P. Dale_, Jun 21 2017 *)

%o (PARI) L(n) = for(x=2,n,y=Li(2*x)-Li(x);print1(floor(y)",")) Li(x) = \ Logarithmic integral { -eint1(log(1/x)) }

%K easy,nonn

%O 2,2

%A _Cino Hilliard_, Jul 22 2005