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A237657 a(n) = |{n < m < 2*n: pi(m) and pi(m^2) are both prime}|, where pi(.) is given by A000720. 5

%I #7 Feb 10 2014 22:24:14

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

%T 4,4,4,4,4,4,3,3,3,3,3,3,3,3,3,3,3,3,3,3,4,6,6,6,5,4,4,4,4,5,5,5,5,5,

%U 4,4

%N a(n) = |{n < m < 2*n: pi(m) and pi(m^2) are both prime}|, where pi(.) is given by A000720.

%C Conjecture: (i) a(n) > 0 for all n > 8.

%C (ii) For any integer n > 1 there is a prime p <= n such that n + pi(p) is prime. Also, for n > 5 there is a prime p with n < p < 2*n such that pi(p) is prime.

%C (iii) For each n > 20, there is a prime p with n < p < 2*n such that pi(p^2) is prime.

%H Zhi-Wei Sun, <a href="/A237657/b237657.txt">Table of n, a(n) for n = 1..10000</a>

%e a(4) = 1 since pi(6) = 3 and pi(6^2) = 11 are both prime.

%e a(10) = 1 since pi(17) = 7 and pi(17^2) = 61 are both prime.

%e a(17) = 1 since pi(33) = 11 and pi(33^2) = 181 are both prime.

%t q[n_]:=PrimeQ[PrimePi[n]]&&PrimeQ[PrimePi[n^2]]

%t a[n_]:=Sum[If[q[m],1,0],{m,n+1,2n-1}]

%t Table[a[n],{n,1,70}]

%Y Cf. A000040, A000720, A038107, A237578, A237643, A237656.

%K nonn

%O 1,18

%A _Zhi-Wei Sun_, Feb 10 2014

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Last modified March 28 15:38 EDT 2024. Contains 371254 sequences. (Running on oeis4.)