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 A019279 Superperfect numbers: sigma(sigma(n)) = 2n where sigma is the sum-of-divisors function A000203. 77
 2, 4, 16, 64, 4096, 65536, 262144, 1073741824, 1152921504606846976 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Let sigma_m(n) be result of applying sum-of-divisors function m times to n; call n (m,k)-perfect if sigma_m (n) = k*n; sequence gives (2,2)-perfect numbers. Even values of these are 2^(p-1) where 2^p-1 is a Mersenne prime (A000043 and A000668). No odd superperfect numbers are known. Hunsucker and Pomerance checked that there are no odd ones below 7 * 10^24. - Jud McCranie, Jun 01 2000 The number of divisors of a(n) is equal to A000043(n), if there are no odd superperfect numbers. - Omar E. Pol, Feb 29 2008 The sum of divisors of a(n) is the n-th Mersenne prime A000668(n), provided that there are no odd superperfect numbers. - Omar E. Pol, Mar 11 2008 Largest proper divisor of A072868(n) if there are no odd superperfect numbers. - Omar E. Pol, Apr 25 2008 This sequence is a divisibility sequence if there are no odd superperfect numbers. - Charles R Greathouse IV, Mar 14 2012 For n>1, sigma(sigma(a(n))) + phi(phi(a(n))) = (9/4)*a(n). - Farideh Firoozbakht, Mar 02 2015 LINKS G. L. Cohen and H. J. J. te Riele, Iterating the sum-of-divisors function, Experimental Mathematics, 5 (1996), pp. 93-100. G. G. Dandapat, J. L. Hunsucker, and Carl Pomerance, Some new results on odd perfect numbers, Pacific J. Math. Volume 57, Number 2 (1975), 359-364. A. Hoque, H. Kalita, Generalized perfect numbers connected with arithmetic functions, Math. Sci. Lett. 3, No. 3, 249-253 (2014). Paul Shubhankar, Ten Problems of Number Theory, International Journal of Engineering and Technical Research (IJETR), ISSN: 2321-0869, Volume-1, Issue-9, November 2013 L. Toth, The alternating sum-of-divisors function, 9th Joint Conf. on Math. and Comp. Sci., February 9-12, 2012, Siofok, Hungary. L. Toth, A survey of the alternating sum-of-divisors function, arXiv:1111.4842 [math.NT], 2011-2014. Eric Weisstein's World of Mathematics, Superperfect Number FORMULA a(n) = (1 + A000668(n))/2, if there are no odd superperfect numbers. - Omar E. Pol, Mar 11 2008 Also, if there are no odd superperfect numbers then a(n) = 2^A000043(n)/2 = A072868(n)/2 = A032742(A072868(n)). - Omar E. Pol, Apr 25 2008 a(n) = 2^A090748(n), if there are no odd superperfect numbers. - Ivan N. Ianakiev, Sep 04 2013 EXAMPLE sigma(sigma(4))=2*4, so 4 is in the sequence. MATHEMATICA sigma = DivisorSigma[1, #]&; For[n = 2, True, n++, If[sigma[sigma[n]] == 2 n, Print[n]]] (* Jean-François Alcover, Sep 11 2018 *) PROG (PARI) is(n)=sigma(sigma(n))==2*n \\ Charles R Greathouse IV, Nov 20 2012 CROSSREFS Cf. A019280, A000203, A000396, A000668, A000043, A034897, A061652, A032742, A072868. Sequence in context: A271234 A061286 A288756 * A061652 A278913 A306979 Adjacent sequences:  A019276 A019277 A019278 * A019280 A019281 A019282 KEYWORD nonn,more,nice AUTHOR EXTENSIONS a(8)-a(9) from Jud McCranie, Jun 01 2000 Corrected by Michel Marcus, Oct 28 2017 STATUS approved

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Last modified October 14 04:44 EDT 2019. Contains 327995 sequences. (Running on oeis4.)