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A235989 sigma(n) is an additive inverse of n modulo phi(n). 1
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 (list; graph; refs; listen; history; text; internal format)
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

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Last modified June 25 12:55 EDT 2019. Contains 324352 sequences. (Running on oeis4.)