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 A235989 sigma(n) is an additive inverse of n modulo phi(n). 1

%I

%S 1,2,6,10,12,28,76,120,312,588,672,888,1060,1264,1656,14496,17900,

%T 22896,44676,71712,77688,95040,183600,233088,327424,411264,425376,

%U 446016,453258,655776,1041120,1253304,2708640,5241856,5468352,8676576,9738912,12536640,59489184

%N sigma(n) is an additive inverse of n modulo phi(n).

%C sigma(10) = 18 is congruent to 2 = -10 mod 4 and phi(10) = 4; so 10 is a term of the sequence.

%C If p = 5*2^k-1 is a prime, as it happens for k = 2, 4, 8, 10, 12, 14,... (A001770), then n = 2^k*p is in the sequence, since n+sigma(n) = 6*phi(n). - _Giovanni Resta_, Jan 27 2014

%H Giovanni Resta, <a href="/A235989/b235989.txt">Table of n, a(n) for n = 1..59</a> (terms < 10^11)

%t t = {1}; For[i = 1, i <= 10^6, i++; If[Mod[DivisorSigma[1, i] + i, EulerPhi[i]] == 0, AppendTo[t, i]]]; t

%o (PARI) isok(n) = !((sigma(n) + n) % eulerphi(n)); \\ _Michel Marcus_, Jan 27 2014

%Y Cf. A001770.

%K nonn

%O 1,2

%A _Joseph L. Pe_, Jan 27 2014

%E More terms from _Michel Marcus_, Jan 27 2014

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Last modified July 20 11:28 EDT 2019. Contains 325180 sequences. (Running on oeis4.)