A100915 Numbers n such that n plus n-th semiprime is semiprime.

4, 6, 9, 12, 16, 18, 19, 20, 24, 29, 31, 34, 35, 39, 40, 44, 46, 49, 51, 54, 55, 72, 73, 76, 79, 80, 81, 84, 87, 91, 93, 94, 96, 98, 110, 113, 116, 120, 128, 130, 136, 137, 148, 150, 154, 159, 165, 168, 170, 172, 175, 188, 190, 191, 199, 200, 206, 215, 217, 220, 230

Offset: 1

Author

Ray Chandler, Nov 26 2004

Keywords

• easy
• nonn

Comments

This is the semiprime analog of A064402.

Examples

			a(3) = 9 because 9 + semiprime(9) = 9 + 25 = 34 is semiprime.
		

Links

• Harvey P. Dale, Table of n, a(n) for n = 1..1000
• Eric Weisstein, World of Mathematics, Semiprime.

Crossrefs

Cf. A001358, A064402, A100493, A100466, A100467, A100916.

Programs

Mathematica

With[{c=Select[Range[1000],PrimeOmega[#]==2&]},Transpose[Select[Thread[ {c,Range[ Length[c]]}], PrimeOmega[Total[#]]==2&]][[2]]] (* Harvey P. Dale, Oct 25 2011 *)

Formula

a(n) = A100466(n) - A100916(n) = A100466(n) - A001358(A100915(n)).