A113784 Difference between semiprime(n) and semiprime(n+2).

5, 4, 5, 5, 7, 7, 4, 4, 8, 8, 2, 4, 4, 8, 10, 5, 6, 6, 3, 5, 7, 7, 9, 8, 8, 8, 4, 2, 5, 6, 3, 2, 12, 16, 9, 7, 4, 3, 3, 2, 7, 10, 5, 8, 8, 2, 3, 3, 10, 12, 4, 3, 7, 8, 11, 9, 6, 7, 4, 9, 14, 8, 2, 3, 3, 4, 7, 5, 2, 3, 3, 2, 3, 7, 14, 11, 12, 12, 6, 5, 6, 8, 6, 5, 9, 11, 13, 11, 4, 6, 7, 4, 3, 3, 2, 3, 6, 9

Offset: 1

Author

Jonathan Vos Post, Jan 20 2006

Keywords

• easy
• nonn

Comments

Semiprime analog of A031131 "Difference between n-th prime and (n+2)nd prime."

Examples

			a(1) = 5 because 3rd semiprime - first semiprime = 9 - 4 = 5.
a(2) = 4 because semiprime(4) - semiprime(2) = 10 - 6 = 4.
a(3) = 5 because semiprime(5) - semiprime(3) = 14 - 9 = 5.
a(4) = 5 because semiprime(6) - semiprime(4) = 15 - 10 = 5.
		

Crossrefs

Cf. A001358, A056809.

Programs

Mathematica

t = Select[ Range@320, Plus @@ Last /@ FactorInteger@# == 2 &]; Drop[t, 2] - Drop[t, -2] (* Robert G. Wilson v *)

Formula

a(n) = A001358(n+2) - A001358(n).

Extensions

More terms from Robert G. Wilson v, Jan 21 2006