A117880 a(1) = 4; a(n) is smallest semiprime > 2*a(n-1).

4, 9, 21, 46, 93, 187, 377, 755, 1513, 3027, 6059, 12127, 24257, 48529, 97059, 194127, 388257, 776515, 1553033, 3106083, 6212177, 12424355, 24848723, 49697447, 99394909, 198789819, 397579639, 795159283, 1590318573, 3180637153

Offset: 1

Author

Jonathan Vos Post, May 04 2006

Keywords

• easy
• nonn
• less

Comments

a(1)=4, a(n)=2*a(n-1)+k, where k is least positive integer chosen so that a(n) is the product of two primes. Corresponding k's are 1, 3, 4, 1, 1, 3, 1, 3, 1, 5, 9, 3, 15, 1, 9, 3, 1, 3, 17, 11, 1, 13, 1, 15, 1, 1, 5, 7, 7, 11, 5, 5, 15, 1, 3, 9, 9, 5, 7, 8, ... - Zak Seidov, Dec 24 2007

Examples

			a(1)=4, then
k=1, a(2)=2*4+1=9,
k=3, a(3)=2*9+3=21,
k=4, a(4)=2*21+4=46,
k=1, a(5)=2*46+1=93,
k=1, a(6)=2*93+1=187.
		

Crossrefs

Semiprime analog of A055496.

Cf. A001358, A076656.

Programs

Mathematica

a=0;Do[Do[b=2a+n;If[2==Plus@@FactorInteger[b][[All,2]],Print[b];Break[]],{n,1000}];a=b,{40}] (* Zak Seidov, Dec 24 2007 *)
ssp[n_]:=Module[{k=2n+1},While[PrimeOmega[k]!=2,k++];k]; NestList[ssp,4,30] (* Harvey P. Dale, Apr 14 2022 *)

Extensions

Edited by N. J. A. Sloane, Jul 01 2008 at the suggestion of R. J. Mathar