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A037037
Number of primes between n and 3n.
1
2, 3, 3, 3, 4, 4, 5, 5, 5, 6, 7, 6, 7, 7, 8, 9, 9, 9, 9, 9, 10, 10, 11, 11, 12, 12, 13, 14, 14, 14, 14, 13, 14, 15, 16, 17, 18, 18, 18, 18, 18, 17, 18, 18, 18, 19, 20, 19, 19, 20, 21, 21, 22, 21, 22, 23, 23, 24, 24, 24, 25, 24, 24, 25, 26, 27, 28, 27, 27, 27, 28, 27, 27, 26, 27, 28
OFFSET
1,1
FORMULA
a(n) = A000720(3n) - A000720(n-1). - Wesley Ivan Hurt, Jun 15 2013
EXAMPLE
For example a(5)=4 because between 5 and 15 there are 4 primes: 5, 7, 11 and 13.
MATHEMATICA
Array[Count[Range[#, 3 #], k_ /; PrimeQ@ k] &, 76] (* Michael De Vlieger, Mar 20 2017 *)
PROG
(PARI) a(n) = primepi(3*n) - primepi(n-1); \\ Michel Marcus, Sep 28 2013
CROSSREFS
Cf. A014085.
Sequence in context: A293137 A087823 A230418 * A156262 A218447 A305845
KEYWORD
nonn
AUTHOR
EXTENSIONS
More terms from Erich Friedman
STATUS
approved