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A323135 a(n) is the least number of iterations that n requires to reach a power of a prime under the map x -> A070229(x), or -1 if we never reach a power of a prime. 2
0, 0, 0, 0, 0, 1, 0, 0, 0, 3, 0, 3, 0, 5, 2, 0, 0, 5, 0, 1, 4, 9, 0, 1, 0, 11, 0, 3, 0, 3, 0, 0, 8, 15, 2, 11, 0, 17, 10, 9, 0, 1, 0, 7, 8, 21, 0, 15, 0, 7, 14, 9, 0, 17, 6, 7, 16, 27, 0, 9, 0, 29, 6, 0, 8, 5, 0, 13, 20, 5, 0, 15, 0, 35, 14, 15, 4, 7, 0, 13, 0 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,10

COMMENTS

The powers of primes correspond to A000961.

Apparently, a(n) >= 0 for any n > 0.

For a given number n, while iterating A070229, we may encounter several prime increments (=several greatest prime factors). It is likely that the number of distinct increments before reaching a power of a prime is not bounded.

For k = 0..9, the least numbers with k distinct increments are:

  k  n       Increments

  -  ------  ------------------------------------------

  0       1  {}

  1       6  {3}

  2      12  {3, 5}

  3      72  {3, 5, 17}

  4     135  {5, 7, 11, 17}

  5     686  {7, 11, 13, 19, 41}

  6   12408  {47, 53, 59, 71, 89, 149}

  7   35378  {19, 23, 67, 89, 101, 179, 211}

  8  127581  {43, 53, 73, 103, 113, 227, 283, 457}

  9  222111  {37, 79, 97, 191, 233, 239, 311, 359, 631}

LINKS

Rémy Sigrist, Table of n, a(n) for n = 1..10000

FORMULA

a(n) = 0 iff n belongs to A000961.

EXAMPLE

For n = 12:

- 12 = 2^2 * 3,

- A070229(12) = 12 + 3 = 15 = 3 * 5,

- A070229(15) = 15 + 5 = 20 = 2^2 * 5,

- A070229(20) = 20 + 5 = 25 = 5^5,

- hence we need at least 3 iterations of A070229 to reach a power of a prime,

- and a(12) = 3.

PROG

(PARI) a(n) = for (k=0, oo, if (omega(n) <= 1, return (k), my (f=factor(n)); n += f[#f~, 1]))

CROSSREFS

See A323136 for the corresponding powers of primes.

Cf. A000961, A070229.

Sequence in context: A291971 A240923 A272727 * A100258 A045763 A132748

Adjacent sequences:  A323132 A323133 A323134 * A323136 A323137 A323138

KEYWORD

nonn

AUTHOR

Rémy Sigrist, Jan 05 2019

STATUS

approved

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Last modified June 16 06:46 EDT 2019. Contains 324145 sequences. (Running on oeis4.)