A156321 a(1)=0, a(n+1) is smallest nonprime > 2*a(n).

0, 1, 4, 9, 20, 42, 85, 171, 343, 687, 1375, 2751, 5504, 11009, 22019, 44039, 88080, 176162, 352325, 704651, 1409303, 2818607, 5637215, 11274431, 22548863, 45097727, 90195455, 180390911, 360781823, 721563647, 1443127295, 2886254591

Offset: 1

Author

Juri-Stepan Gerasimov, Feb 08 2009

Keywords

• nonn

Examples

			a(5) is the smallest nonprime > 2*a(4)=2*9=18, hence a(5)=20.
		

Links

• Harvey P. Dale, Table of n, a(n) for n = 1..1000

Crossrefs

Cf. A000027, A055496, A141468.

Programs

Mathematica

np[n_]:=Module[{k=2n+1},If[PrimeQ[k],k+1,k]]; NestList[np,0,40] (* Harvey P. Dale, May 23 2012 *)

Extensions

More terms from R. J. Mathar, Feb 10 2009