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 A211677 First number k whose value of sigma(k)/k appears n times. 2
 1, 28, 496, 1638, 24384, 2886100, 13035330, 29410290, 4426793280 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS The values of sigma(k)/k are 1, 2, 2, 8/3, 8/3, 96/35, 32/9, 32/9, 32/7. Note that these values are nondecreasing. Is that always the case? In the table below, all numbers in the same row are friendly to each other. a(10) <= 27477725184. a(11) <= 88071903612. a(12) <= A027687(12). - Donovan Johnson, Aug 06 2012 For n>1, these are the smallest numbers to appear consecutively (n-1) times in A050973. - Michel Marcus, Jan 28 2014 LINKS Table of n, a(n) for n=1..9. Claude W. Anderson and Dean Hickerson, Problem 6020: Friendly Integers, Amer. Math. Monthly 84 (1977) pp. 65-66. Achim Flammenkamp, Multiply Perfect Numbers (sigma(k)/k is an integer) Tom De Medts, MathOverflow: Recovering n from sigma(n)/n Carl Pomerance, Multiply perfect numbers, Mersenne primes and effective computability, Math. Ann. 226 (1977), 195-206. Eric W. Weisstein, Friendly number Eric W. Weisstein, Friendly pair Wikipedia, Friendly number EXAMPLE These are the values of k such that sigma(k)/k appears n times: n k values 1: 1 2: 6, 28 3: 6, 28, 496 4: 84, 270, 1488, 1638 5: 84, 270, 1488, 1638, 24384 6: 210, 17360, 43400, 284480, 2229500, 2886100 7: 3780, 66960, 167400, 406224, 1097280, 6656832, 13035330 8: 3780, 66960, 167400, 406224, 1097280, 6656832, 13035330, 29410290 9: 164989440, 270138960, 318729600, 326781000, 481572000, 623397600, 675347400, 995248800, 4426793280 - Donovan Johnson, Aug 06 2012 These numbers appear in A211679. CROSSREFS Cf. A000203 (sigma), A050973, A211679. Sequence in context: A240463 A140107 A028170 * A076172 A004336 A079598 Adjacent sequences: A211674 A211675 A211676 * A211678 A211679 A211680 KEYWORD nonn,hard,more AUTHOR T. D. Noe, May 09 2012 EXTENSIONS a(7)-a(8) from Donovan Johnson, May 10 2012 a(9) from Michel Marcus and Donovan Johnson, Aug 06 2012 STATUS approved

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Last modified June 19 11:49 EDT 2024. Contains 373503 sequences. (Running on oeis4.)