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

1,2

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)

Mathematica

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

Prog

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

Crossrefs

Cf. A001770.

Sequence in context: A140776 A277238 A108783 * A066679 A086123 A144031

Adjacent sequences: A235986 A235987 A235988 * A235990 A235991 A235992

Keyword

nonn

Author

Joseph L. Pe, Jan 27 2014

Extensions

More terms from Michel Marcus, Jan 27 2014

Status

approved