A165802 Primes p with f(p), f(f(p)), ... all prime, where f(n) = (n-1)/2. Stop when f(...f(p)...) is less than 4.

2, 3, 5, 7, 11, 23, 47

Offset

1,1

Links

Table of n, a(n) for n=1..7.

Example

(47-1)/2=23(prime);(23-1)/2=11(prime); (11-1)/2=5(prime); (5-1)/2=2.

Mathematica

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

Crossrefs

Subsequence of A165801.

Sequence in context: A252791 A068148 A036344 * A107798 A119660 A079148

Adjacent sequences: A165799 A165800 A165801 * A165803 A165804 A165805

Keyword

nonn,fini,full

Author

Vladimir Joseph Stephan Orlovsky, Sep 28 2009

Extensions

Edited by Charles R Greathouse IV, Oct 05 2009

Status

approved