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A221205 a(n) is the nearest integer to sqrt(pi(10^n)) (see A006880). 3
0, 2, 5, 13, 35, 98, 280, 815, 2400, 7131, 21332, 64172, 193928, 588273, 1790235, 5463018, 16710426, 51220671, 157289397, 483795067, 1490241458, 4596440959, 14193917243, 43878472986, 135777758736, 420530985064, 1303551591182, 4043817556078, 12553456467283, 38996129419652 (list; graph; refs; listen; history; text; internal format)
OFFSET
0,2
COMMENTS
Nearest integer to sqrt(pi(10^n)) where pi(10^n) is the number of primes < 10^n (A006880).
LINKS
FORMULA
a(n) = round(sqrt(A006880(n))).
MATHEMATICA
Array[Round@ Sqrt@ PrimePi[10^#] &, 17, 0] (* Michael De Vlieger, Mar 19 2021 *)
PROG
(PARI) a(n) = round(sqrt(primepi(10^n))); \\ Michel Marcus, Mar 19 2021
(Python)
from math import isqrt
from sympy import primepi
def A221205(n): return (m:=isqrt(k:=int(primepi(10**n))))+int(k-m*(m+1)>=1) # Chai Wah Wu, Jul 31 2022
CROSSREFS
Cf. A006880.
Sequence in context: A086581 A059027 A025198 * A037247 A308273 A126221
KEYWORD
nonn
AUTHOR
Vladimir Pletser, Jan 04 2013
EXTENSIONS
a(21) to a(24) added by Vladimir Pletser, Feb 27 2013 (recovered by Georg Fischer, Jan 20 2019)
a(25)-a(28), obtained using A006880, added by Eduard Roure Perdices, Apr 16 2021
a(29), obtained using A006880, added by Chai Wah Wu, Jul 31 2022
STATUS
approved

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Last modified March 18 22:56 EDT 2024. Contains 370952 sequences. (Running on oeis4.)