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 A247086 Integers n such that numerator and denominator of sigma(n)/n are both prime. 1
 2, 12, 24, 90, 234, 528, 588, 1456, 4320, 4680, 9024, 11466, 26208, 35640, 55104, 55552, 90816, 114660, 121024, 123648, 185562, 199584, 297600, 510720, 674880, 2142720, 2190336, 4112640, 4316928, 5322240, 5659641, 7600320, 8714160, 10281600, 12999168, 15268368 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 LINKS Michel Marcus, Table of n, a(n) for n = 1..81 EXAMPLE For n=2, sigma(n)/n = 3/2 with both numerator and denominator prime. MATHEMATICA a247086Q[n_] := Block[{f = DivisorSigma[1, n]/n}, And[PrimeQ@ Numerator@ f, PrimeQ@ Denominator@ f]]; a247086[n_] := Select[Range@ n, a247086Q@ # &]; a247086[10^6] (* Michael De Vlieger, Jan 11 2015 *) PROG (PARI) isok(n) = my(ab = sigma(n)/n); isprime(numerator(ab)) && isprime(denominator(ab)); CROSSREFS Cf. A000203 (sigma(n)). Cf. A017665 (numerator of sigma(n)/n), A017666 (denominator of sigma(n)/n). Sequence in context: A176679 A278407 A224923 * A121119 A226899 A052565 Adjacent sequences:  A247083 A247084 A247085 * A247087 A247088 A247089 KEYWORD nonn AUTHOR Michel Marcus, Jan 10 2015 STATUS approved

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Last modified June 18 18:52 EDT 2019. Contains 324215 sequences. (Running on oeis4.)