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A039655 Number of iterations of f(x) = sigma(x)-1 applied to n required to reach a prime, or -1 if no prime is ever reached. 14
0, 0, 2, 0, 1, 0, 2, 5, 1, 0, 4, 0, 1, 1, 2, 0, 2, 0, 1, 1, 2, 0, 1, 2, 1, 3, 2, 0, 1, 0, 5, 1, 1, 1, 2, 0, 1, 2, 1, 0, 4, 0, 1, 5, 1, 0, 2, 4, 2, 1, 1, 0, 3, 1, 3, 1, 1, 0, 1, 0, 4, 1, 2, 1, 2, 0, 3, 4, 2, 0, 2, 0, 1, 2, 1, 4, 1, 0, 2, 2, 3, 0, 1, 1, 1, 3, 1, 0, 1, 2, 1, 1, 2, 3, 1, 0, 3, 2, 2, 0, 2, 0, 2, 1, 2 (list; graph; refs; listen; history; text; internal format)
OFFSET
2,3
LINKS
Charles R Greathouse IV, Table of n, a(n) for n = 2..10000
Hugo Pfoertner, Terms a(2)...a(1000000).
N. J. A. Sloane, Three (No, 8) Lovely Problems from the OEIS, Experimental Mathematics Seminar, Rutgers University, Oct 05 2017, Part I, Part 2, Slides. (Mentions this sequence)
MATHEMATICA
f[n_] := Plus @@ Divisors@n - 1; g[n_] := Length@ NestWhileList[ f@# &, n, !PrimeQ@# &] - 1; Table[ g@n, {n, 2, 106}] (* Robert G. Wilson v, May 07 2010 *)
PROG
(PARI) a(n)=my(t); while(!isprime(n), n=sigma(n)-1; t++); t \\ Charles R Greathouse IV, Sep 16 2014
CROSSREFS
For records see A292114 and A292115.
Sequence in context: A163577 A132178 A357869 * A357882 A103775 A331594
KEYWORD
nonn
AUTHOR
EXTENSIONS
Escape clause added by N. J. A. Sloane, Aug 31 2017
STATUS
approved

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Last modified March 18 21:02 EDT 2024. Contains 370951 sequences. (Running on oeis4.)