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%I #52 Nov 16 2017 02:34:44
%S 2,6,4,27,12,9,121,301,930,484,578,441,1273,468,4863,3171,9216,8373,
%T 19692,19416,25442,13440,19230,16641,16804,83161,100652,226181,203400,
%U 133200,419248,380979,744796,553296,634710,539476,505584,674416,634206
%N Becomes prime after n iterations of f(x) = sigma(x)-1 (least inverse of A039655).
%C 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
%C 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
%C Checked through a(138)=60780636903. - _Hugo Pfoertner_, Nov 15 2017
%H Hugo Pfoertner, <a href="/A039656/b039656.txt">Table of n, a(n) for n = 0..138</a> (terms up to 66 from David W. Wilson, 67-100 from Robert G. Wilson v and Charles R Greathouse IV).
%t 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 *)
%Y Cf. A039649, A039650, A039651, A039652, A039653, A039654, A039655, A292114, A292115.
%K nonn
%O 0,1
%A _David W. Wilson_
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