The OEIS Foundation is supported by donations from users of the OEIS and by a grant from the Simons Foundation.

 Hints (Greetings from The On-Line Encyclopedia of Integer Sequences!)
 A066422 a(n) = least k such that phi^(k)(n) + 1 is prime, where phi^(k) denotes application of phi k times. 1
 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 2, 2, 1, 1, 1, 2, 1, 1, 1, 2, 3, 1, 1, 1, 1, 2, 1, 1, 3, 1, 3, 1, 1, 1, 3, 1, 1, 1, 1, 3, 3, 1, 1, 1, 1, 3, 2, 3, 1, 1, 1, 3, 1, 1, 1, 1, 1, 1, 1, 2, 2, 3, 1, 2, 4, 3, 1, 3, 1, 1, 1, 1, 1, 3, 1, 2 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,15 LINKS Charles R Greathouse IV, Table of n, a(n) for n = 1..10000 EXAMPLE EulerPhi(EulerPhi(15)) + 1 = EulerPhi(8) + 1 = 4 + 1 = 5, a prime; so a(15) = 2. PROG (PARI) a(n) = {nb = 1; n = eulerphi(n); while(! isprime(n+1), n = eulerphi(n); nb ++; ); return (nb); } \\ Michel Marcus, May 18 2013 CROSSREFS Cf. A000010. Sequence in context: A140500 A156054 A030616 * A274888 A239702 A202084 Adjacent sequences:  A066419 A066420 A066421 * A066423 A066424 A066425 KEYWORD nonn AUTHOR Joseph L. Pe, Dec 26 2001 EXTENSIONS Corrected and extended by Michel Marcus, May 18 2013 STATUS approved

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recent
The OEIS Community | Maintained by The OEIS Foundation Inc.

Last modified May 30 15:37 EDT 2020. Contains 334726 sequences. (Running on oeis4.)