A165801 f(n), f(f(n)), ... are all prime, where f(n) = (n-1)/2. Stop when f(...f(n)...) is less than 4.

1, 2, 3, 5, 7, 11, 15, 23, 47, 95

Offset: 1

Author

Vladimir Joseph Stephan Orlovsky, Sep 28 2009

Keywords

• nonn
• easy
• fini
• full

Comments

There are no more terms since 95 is composite.

Examples

			(95-1)/2=47(prime); (47-1)/2=23(prime); (23-1)/2=11(prime); (11-1)/2=5(prime); (5-1)/2=2(prime); no composite numbers smaller than 2 exist, end. (47-1)/2=23(prime);(23-1)/2=11(prime); (11-1)/2=5(prime); (5-1)/2=2; no composite numbers smaller than 2 exist, end.
		

Programs

Mathematica

f[n_]:=Module[{k=n},While[k>3,k=(k-1)/2;If[ !PrimeQ[k],Break[]]];PrimeQ[k]]; lst={};Do[If[f[n],AppendTo[lst,n]],{n,5!}];lst

Extensions

Corrected and edited by Charles R Greathouse IV, Oct 05 2009