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

%I #5 May 23 2012 15:13:42

%S 0,1,4,9,20,42,85,171,343,687,1375,2751,5504,11009,22019,44039,88080,

%T 176162,352325,704651,1409303,2818607,5637215,11274431,22548863,

%U 45097727,90195455,180390911,360781823,721563647,1443127295,2886254591

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

%H Harvey P. Dale, <a href="/A156321/b156321.txt">Table of n, a(n) for n = 1..1000</a>

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

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

%Y Cf. A000027, A055496, A141468.

%K nonn

%O 1,3

%A _Juri-Stepan Gerasimov_, Feb 08 2009

%E More terms from _R. J. Mathar_, Feb 10 2009

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Last modified March 28 16:34 EDT 2024. Contains 371254 sequences. (Running on oeis4.)