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

1,1

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

Links

Table of n, a(n) for n=1..30.

Example

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.

Mathematica

a=4; Do[Do[b=2a+n; If[2==Plus@@FactorInteger[b][[All, 2]], Print[{b, n}]; 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 *)

Crossrefs

Semiprime analog of A055496.

Cf. A001358, A076656.

Sequence in context: A009914 A048638 A144527 * A027973 A348407 A103040

Adjacent sequences: A117877 A117878 A117879 * A117881 A117882 A117883

Keyword

easy,nonn,less

Author

Jonathan Vos Post, May 04 2006

Extensions

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

Status

approved