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

1,2

Comments

There are no more terms since 95 is composite.

Links

Table of n, a(n) for n=1..10.

Example

(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.

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

Crossrefs

Sequence in context: A326591 A177485 A218023 * A239054 A241725 A022480

Adjacent sequences: A165798 A165799 A165800 * A165802 A165803 A165804

Keyword

nonn,easy,fini,full

Author

Vladimir Joseph Stephan Orlovsky, Sep 28 2009

Extensions

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

Status

approved