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Number of primes not greater than n that are greater than the number of primes not greater than n.
4

%I #7 Nov 21 2013 12:49:19

%S 0,1,1,1,1,1,2,2,2,2,2,2,3,3,3,3,3,3,4,4,4,4,5,5,5,5,5,5,6,6,6,6,6,6,

%T 6,6,7,7,7,7,7,7,8,8,8,8,9,9,9,9,9,9,10,10,10,10,10,10,10,10,11,11,11,

%U 11,11,11,11,11,11,11,12,12,13,13,13,13,13,13,14,14,14,14,14,14,14,14

%N Number of primes not greater than n that are greater than the number of primes not greater than n.

%C a(n) = #{p prime: A000720(n)<p<=n} = A000720(n)-A132090(n);

%C a(A007821(n)) = n.

%H R. Zumkeller, <a href="/A137588/b137588.txt">Table of n, a(n) for n = 1..10000</a>

%e n=100: A000720(100)=25,

%e a(100)=#{29,31,37,41,43,47,53,59,61,67,71,73,79,83,89,97}=16.

%t Table[PrimePi[n]-PrimePi[PrimePi[n]],{n,90}] (* _Harvey P. Dale_, Mar 20 2011 *)

%K nonn

%O 1,7

%A _Reinhard Zumkeller_, Jan 28 2008