A175613 Number of semiprimes <= 2^prime(n).

1, 2, 10, 42, 589, 2186, 30253, 113307, 1608668, 88157689, 336717854, 19015826478, 282528883551, 1091574618496, 16360940729894

Offset

1,2

Links

Table of n, a(n) for n=1..15.

Formula

a(n) = A072000(A034785(n)) = A125527(A000040(n)). [R. J. Mathar, Dec 10 2010]

Example

a(2)=2 because first 2 semiprimes are 4, 6 both <2^prime(2)=8.

Mathematica

(* First run program given in A072000 to define the SemiPrimePi function *) Table[SemiPrimePi[2^Prime[n]], {n, 10}](* Alonso del Arte, Dec 10 2010 *)

Prog

(PARI) a(n)=my(N=2^prime(n), s, i); forprime(p=2, sqrtint(N), s+=primepi(N\p); i++); s - i * (i-1)/2 \\ Charles R Greathouse IV, Apr 25 2016

Crossrefs

Cf. A001358, A007053, a proper subset of A125527.

Sequence in context: A119694 A286760 A197048 * A296001 A121949 A005144

Adjacent sequences: A175610 A175611 A175612 * A175614 A175615 A175616

Keyword

nonn,less

Author

Juri-Stepan Gerasimov, Dec 04 2010

Extensions

a(14) & a(15) from Robert G. Wilson v, Oct 19 2011.

Status

approved