A038108 Number of prime pairs {p,q}, such that pq < n^2.

0, 2, 6, 8, 13, 16, 22, 26, 34, 39, 48, 55, 62, 75, 82, 89, 103, 113, 126, 135, 149, 163, 179, 190, 202, 220, 236, 252, 270, 288, 304, 320, 340, 360, 381, 403, 425, 443, 462, 483, 508, 532, 556, 581, 604, 633, 655, 678, 709, 738, 761, 782

Offset: 2

Author

Joe K. Crump (joecr(AT)carolina.rr.com)

Keywords

• nonn

Comments

Number of semiprimes (A001358) < n^2. [Michel Marcus, Sep 02 2013]

Examples

			a(3)=2 because only the prime pairs (2,2) and (2,3) form products < 9.
		

Crossrefs

Cf. A205726.

Programs

PARI

a(n) = {sqn = n^2; idp = primepi(sqn\2); nbp = 0; for (i = 1, idp, p = prime(i); for (j = 1, i, if (p * prime(j) < sqn, nbp++););); nbp;} \\ Michel Marcus, Sep 02 2013