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%I #17 Jan 28 2020 00:29:38
%S 2,3,4,7,8,9,10,19,20,21,22,53,54,55,56,57,58,131,132,133,134,135,136,
%T 311,312,719,720,721,722,723,724,725,726,1619,1620,3671,3672,8161,
%U 8162,8163,8164,8165,8166,17863,17864,17865,17866,17867,17868,17869,17870
%N pi(n) is a power of 2, where pi(n) = A000720(n) is the number of primes <= n.
%C The numbers occur in blocks of consecutive integers: 2, 3-4, 7-10, 19-22, ...; the n-th block starts at the 2^n-th prime (A033844) and ends just before the (2^n + 1)-th prime (A051439).
%H Chai Wah Wu, <a href="/A073798/b073798.txt">Table of n, a(n) for n = 1..1103</a> (n = 1..665 from Ivan Neretin)
%e 10 is in the sequence since pi(10)=4=2^2.
%t pow2[n_] := n==1||(n>1&&IntegerQ[n/2]&&pow2[n/2]); Select[Range[20000], pow2[PrimePi[ # ]]&]
%t Flatten@Table[Range[p = Prime[2^k], NextPrime[p] - 1], {k, 0, 11}] (* _Ivan Neretin_, Jan 21 2017 *)
%o (PARI) isok(n) = my(pi = primepi(n)); (pi==1) || (pi==2) || (ispower(primepi(n),,&k) && (k==2)); \\ _Michel Marcus_, Jan 23 2017
%Y Cf. A000079, A000720, A015910, A073797, A073799.
%K nonn
%O 1,1
%A _Labos Elemer_, Aug 14 2002
%E Edited by _Dean Hickerson_, Aug 15 2002
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