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A162923 Numbers n such that the equation sigma(sigma(x)) = n has no solutions. 2

%I

%S 2,3,5,6,9,10,11,13,16,17,18,19,20,21,22,23,25,26,27,29,30,31,33,34,

%T 35,36,37,38,40,41,43,44,45,46,47,48,49,50,51,52,53,54,55,57,58,59,61,

%U 62,64,65,66,67,68,69,70,71,73,74,75,76,77,78,79,81,82,83,85,86,87,88,89,92,93

%N Numbers n such that the equation sigma(sigma(x)) = n has no solutions.

%H Ivan Neretin, <a href="/A162923/b162923.txt">Table of n, a(n) for n = 1..10000</a>

%F {A000027} minus {A070286}.

%t mx = 100; Complement[Range[mx], Table[Nest[DivisorSigma[1, #] &, n, 2], {n, mx}]] (* _Ivan Neretin_, Feb 12 2018 *)

%Y Complement of A070286.

%K nonn

%O 1,1

%A _Jaroslav Krizek_, Jul 17 2009

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Last modified January 29 14:23 EST 2020. Contains 331338 sequences. (Running on oeis4.)