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A165801 f(n), f(f(n)), ... are all prime, where f(n) = (n-1)/2. Stop when f(...f(n)...) is less than 4. 3
1, 2, 3, 5, 7, 11, 15, 23, 47, 95 (list; graph; refs; listen; history; text; internal format)
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

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Last modified December 13 23:14 EST 2019. Contains 329974 sequences. (Running on oeis4.)