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

1, 2, 6, 10, 12, 28, 76, 120, 312, 588, 672, 888, 1060, 1264, 1656, 14496, 17900, 22896, 44676, 71712, 77688, 95040, 183600, 233088, 327424, 411264, 425376, 446016, 453258, 655776, 1041120, 1253304, 2708640, 5241856, 5468352, 8676576, 9738912, 12536640, 59489184

Offset: 1

Author

Joseph L. Pe, Jan 27 2014

Keywords

• nonn

Comments

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

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

Links

• Giovanni Resta, Table of n, a(n) for n = 1..59 (terms < 10^11)

Crossrefs

Cf. A001770.

Programs

Mathematica

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

PARI

isok(n) = !((sigma(n) + n) % eulerphi(n)); \\ Michel Marcus, Jan 27 2014

Extensions

More terms from Michel Marcus, Jan 27 2014