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A039656 Becomes prime after n iterations of f(x) = sigma(x)-1 (least inverse of A039655). 9
2, 6, 4, 27, 12, 9, 121, 301, 930, 484, 578, 441, 1273, 468, 4863, 3171, 9216, 8373, 19692, 19416, 25442, 13440, 19230, 16641, 16804, 83161, 100652, 226181, 203400, 133200, 419248, 380979, 744796, 553296, 634710, 539476, 505584, 674416, 634206 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,1

COMMENTS

Records: 2, 6, 27, 121, 301, 930, 1273, 4863, 9216, 19692, 25442, 83161, 100652, 226181, 419248, 744796, 3739690, 4238314, etc. - Robert G. Wilson v, Sep 23 2017

Indices of records: 0, 1, 3, 6, 7, 8, 12, 14, 16, 18, 20, 25, 26, 27, 30, 32, 46, 47, 48, 49, 50, 56, 57, 58, 59, 61, 63, 65, 67, 76, 77, 78, 82, 83, 84, 85, etc. - Robert G. Wilson v, Sep 23 2017

Checked through a(138)=60780636903. - Hugo Pfoertner, Nov 15 2017

LINKS

Hugo Pfoertner, Table of n, a(n) for n = 0..138 (terms up to 66 from David W. Wilson, 67-100 from Robert G. Wilson v and Charles R Greathouse IV).

MATHEMATICA

f[n_] := Plus @@ Divisors@ n -1; g[n_] := Length@ NestWhileList[f@# &, n, !PrimeQ@# &] - 1; t[_] := -1; k = 2; While[k < 10000001, a = g[k]; If[ t@ a == -1, t@ a = k; Print[{a, k}]]; k++]; t@# & /@ Range[0, 50] (* Robert G. Wilson v, Sep 22 2017 *)

CROSSREFS

Cf. A039649, A039650, A039651, A039652, A039653, A039654, A039655, A292114, A292115.

Sequence in context: A106831 A038212 A092399 * A263326 A226532 A006233

Adjacent sequences:  A039653 A039654 A039655 * A039657 A039658 A039659

KEYWORD

nonn

AUTHOR

David W. Wilson

STATUS

approved

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Last modified November 14 14:35 EST 2018. Contains 317208 sequences. (Running on oeis4.)