A144741 Number of semiprimes less than or equal to and not coprime to n.

0, 0, 0, 1, 0, 2, 0, 2, 2, 3, 0, 4, 0, 4, 4, 4, 0, 6, 0, 5, 5, 5, 0, 8, 3, 6, 4, 7, 0, 10, 0, 6, 6, 7, 6, 11, 0, 8, 7, 11, 0, 14, 0, 9, 9, 9, 0, 14, 4, 12, 8, 10, 0, 15, 7, 12, 9, 10, 0, 20, 0, 11, 11, 11, 8, 19, 0, 12, 10, 18, 0, 19, 0, 12, 14, 13, 8, 21, 0, 17, 9, 13, 0, 24, 9, 14, 11, 17, 0, 28, 9

Offset: 1

Author

Reikku Kulon, Sep 20 2008

Keywords

• easy
• nonn

Examples

			a(10) = 3: four, six and ten are semiprimes; nine is a semiprime relatively prime to ten.
		

Crossrefs

Cf. A000010, A001358, A144740.

Programs

PARI

a(n) = sum(k=1, n, if (bigomega(k) == 2, gcd(k,n) != 1)); \\ Michel Marcus, Apr 15 2023