A108216 Number of semiprimes between 10n and 10n + 9.

3, 3, 4, 5, 2, 4, 3, 2, 4, 4, 1, 4, 4, 2, 5, 3, 3, 2, 3, 1, 6, 6, 2, 2, 2, 3, 3, 2, 2, 4, 5, 2, 5, 3, 2, 2, 3, 2, 3, 5, 2, 4, 2, 1, 3, 4, 2, 3, 4, 2, 3, 5, 3, 4, 3, 4, 3, 2, 4, 2, 0, 2, 4, 3, 1, 1, 3, 3, 4, 5, 3, 3, 2, 3, 3, 3, 3, 3, 3, 4, 3, 4, 0, 3, 4, 1, 4, 3, 2, 4, 2, 3, 4, 3, 2, 4, 2, 3, 3, 3, 3, 2, 1, 1, 5

Offset: 0

Author

Giovanni Teofilatto, Jun 16 2005

Keywords

• nonn

Comments

a(60) = a(82) = a(142) = 0. - Jonathan Vos Post, Jun 16 2005

Examples

			a(0) = 3 because between 0 and 9 there are three semiprimes: 4, 6 and 9.
a(1) = 3 because between 10 and 19 there are three semiprimes: 10, 14 and 15.
		

Crossrefs

Cf. A038800 number of primes between 10n and 10n+9.

Cf. A064911, A066265, A072000.

Programs

Mathematica

f[n_] := Sum[ PrimePi[n/Prime[i]] - (i - 1), {i, PrimePi[ Sqrt[n]]}]; Table[f[10n + 9] - f[Max[10n - 1, 0]], {n, 0, 104}] (* Robert G. Wilson v, Ray Chandler *)

Extensions

Edited and extended by Ray Chandler, Jul 07 2005