|
%I #37 Jul 31 2022 14:07:26
%S 0,2,5,13,35,98,280,815,2400,7131,21332,64172,193928,588273,1790235,
%T 5463018,16710426,51220671,157289397,483795067,1490241458,4596440959,
%U 14193917243,43878472986,135777758736,420530985064,1303551591182,4043817556078,12553456467283,38996129419652
%N a(n) is the nearest integer to sqrt(pi(10^n)) (see A006880).
%C Nearest integer to sqrt(pi(10^n)) where pi(10^n) is the number of primes < 10^n (A006880).
%F a(n) = round(sqrt(A006880(n))).
%t Array[Round@ Sqrt@ PrimePi[10^#] &, 17, 0] (* _Michael De Vlieger_, Mar 19 2021 *)
%o (PARI) a(n) = round(sqrt(primepi(10^n))); \\ _Michel Marcus_, Mar 19 2021
%o (Python)
%o from math import isqrt
%o from sympy import primepi
%o def A221205(n): return (m:=isqrt(k:=int(primepi(10**n))))+int(k-m*(m+1)>=1) # _Chai Wah Wu_, Jul 31 2022
%Y Cf. A006880.
%K nonn
%O 0,2
%A _Vladimir Pletser_, Jan 04 2013
%E a(21) to a(24) added by _Vladimir Pletser_, Feb 27 2013 (recovered by _Georg Fischer_, Jan 20 2019)
%E a(25)-a(28), obtained using A006880, added by _Eduard Roure Perdices_, Apr 16 2021
%E a(29), obtained using A006880, added by _Chai Wah Wu_, Jul 31 2022
|
