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A067780 Numbers n such that sigma(n)/phi(n) is prime. 1
2, 3, 12, 15, 35, 56, 78, 140, 190, 357, 630, 812, 910, 1045, 1672, 2214, 2580, 3339, 4180, 4522, 4674, 5049, 5278, 5544, 8008, 10659, 11160, 12441, 16065, 16120, 16770, 18630, 18810, 24244, 24871, 24969, 25070, 25758, 27000, 27170, 28896 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

LINKS

Vincenzo Librandi, Table of n, a(n) for n = 1..600

EXAMPLE

sigma(12)/phi(12) = 28/4 = 7, a prime, so 12 is a term of the sequence.

MATHEMATICA

Select[Range[10^5], PrimeQ[DivisorSigma[1, # ]/EulerPhi[ # ]] &]

CROSSREFS

Sequence in context: A090512 A076175 A289870 * A290168 A124486 A260908

Adjacent sequences:  A067777 A067778 A067779 * A067781 A067782 A067783

KEYWORD

nonn

AUTHOR

Joseph L. Pe, Feb 06 2002

STATUS

approved

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Last modified September 22 15:04 EDT 2021. Contains 347607 sequences. (Running on oeis4.)