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 A007539 a(n) = first n-fold perfect (or n-multiperfect) number. (Formerly M4297) 20
 1, 6, 120, 30240, 14182439040, 154345556085770649600, 141310897947438348259849402738485523264343544818565120000 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS On the Riemann Hypothesis, a(n) > exp(exp(n / e^gamma)) for n > 3. Unconditionally, a(n) > exp(exp(0.9976n / e^gamma)) for n > 3, where the constant is related to A004394(1000000). - Charles R Greathouse IV, Sep 06 2012 Each of the terms 1, 6, 120, 30240 divides all larger terms <= a(8). See A227765, A227766, ..., A227769. - Jonathan Sondow, Jul 30 2013 Is a(n) < a(n+1)? - Jeppe Stig Nielsen, Jun 16 2015 Equivalently, a(n) is the smallest number k such that sigma(k)/k = n. - Derek Orr, Jun 19 2015 The number of divisors of these terms are: 1, 4, 16, 96, 1920, 110592, 1751777280, 63121588161085440. - Michel Marcus, Jun 20 2015 Given n, let S_n be the sequence of integers k that satisfy numerator(sigma(k)/k) = n. Then a(n) is a member of S_n. In fact a(n) = S_n(i), where the successive values of i are 1, 1, 2, 2, 4, 2, (23, 6, 31, 12, ...), where the terms in parentheses need to be confirmed. - Michel Marcus, Nov 22 2015 REFERENCES A. H. Beiler, Recreations in the Theory of Numbers, Dover, NY, 1964, p. 22. A. Brousseau, Number Theory Tables. Fibonacci Association, San Jose, CA, 1973, p. 138. R. K. Guy, Unsolved Problems in Number Theory, B2. N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence). LINKS T. D. Noe, Table of n, a(n) for n = 1..8 Abiodun E. Adeyemi, A Study of @-numbers, arXiv:1906.05798 [math.NT], 2019. C. K. Caldwell, The Prime Glossary, multiply perfect F. Firoozbakht, M. F. Hasler, Variations on Euclid's formula for Perfect Numbers, JIS 13 (2010) #10.3.1. Achim Flammenkamp, The Multiply Perfect Numbers Page Fred Helenius, Link to Glossary and Lists G. P. Michon, Multiperfect and hemiperfect numbers Walter Nissen, Abundancy: Some Resources MATHEMATICA Table[k = 1; While[DivisorSigma[1, k]/k != n, k++]; k, {n, 4}] (* Michael De Vlieger, Jun 20 2015 *) PROG (PARI) a(n)=k=1; while((sigma(k)/k)!=n, k++); k vector(4, n, a(n)) \\ Derek Orr, Jun 19 2015 CROSSREFS Cf. A000396, A005820, A027687, A046060, A046061, A007691, A072002, A227765, A227766, A227767, A227768, A227769. Sequence in context: A012475 A053777 A023199 * A040996 A110442 A137149 Adjacent sequences:  A007536 A007537 A007538 * A007540 A007541 A007542 KEYWORD nonn,nice,more AUTHOR EXTENSIONS More terms sent by Robert G. Wilson v, Nov 30 2000 STATUS approved

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Last modified September 24 20:03 EDT 2020. Contains 337321 sequences. (Running on oeis4.)