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 A330598 Numbers k such that the denominator of sigma(sigma(k))/k is equal to 2. 4
 30, 2046, 245760, 301056, 450560, 1171456, 1351680, 3514368, 14515200, 16760832, 19611648, 77220864, 159373824, 357291648, 391444480, 477216768, 555714432, 754928640, 765414240, 1006602240, 1761500160, 2330913312, 4314834944, 8369053056, 20449394784, 37949317120 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Although the definition here is similar to the one in A019278, it appears that this sequence does not have the same nice features as A019278. Otherwise said: sigma(sigma(k))/k is half-integer, or: sigma(sigma(k)) is an odd multiple of k/2. This also implies that all terms are even. - M. F. Hasler, Jan 06 2020 LINKS Giovanni Resta, Table of n, a(n) for n = 1..38 (terms < 10^13) Michel Marcus, Unexhaustive list of terms EXAMPLE sigma(sigma(30))/30 = sigma(72)/30 = 195/30 = 13/2 so 30 is a term. PROG (PARI) isok(n) = denominator(sigma(sigma(n))/n) == 2; CROSSREFS Cf. A019278 (denominator is 1), A051027 (sigma(sigma)). Cf. A000203 (sigma), A159907 (hemiperfect numbers). Sequence in context: A280670 A092608 A241128 * A255958 A092617 A295446 Adjacent sequences:  A330595 A330596 A330597 * A330599 A330600 A330601 KEYWORD nonn AUTHOR Michel Marcus, Dec 19 2019 EXTENSIONS a(22)-a(26) from Giovanni Resta, Dec 20 2019 STATUS approved

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Last modified September 22 14:37 EDT 2020. Contains 337291 sequences. (Running on oeis4.)