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 A007593 2-hyperperfect numbers: n = 2*(sigma(n) - n - 1) + 1. (Formerly M5121) 13
 21, 2133, 19521, 176661, 129127041, 328256967373616371221, 67585198634817522935331173030319681, 443426488243037769923934299701036035201 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS For all k in A014224, 3^(k-1)*(3^k-2) is in this sequence. - M. F. Hasler, Apr 25 2012 The known examples are all of the form 3^(k-1)*(3^k-2), where 3^k-2 is prime (cf. A014224). Conversely, from sigma(3^(k-1)*p)=(3^k-1)/2*(p+1) it is immediate that 2*sigma(n)=3n+1 for such numbers, i.e., they are 2-hyperperfect. (This is "form 3" with p=3 in McCranie's paper.) - M. F. Hasler, Apr 25 2012 a(6) > 4*10^12. - Donovan Johnson, Nov 20 2012 REFERENCES J.-M. De Koninck, Ces nombres qui nous fascinent, Entry 21, p. 7, Ellipses, Paris 2008. R. K. Guy, Unsolved Problems in Number Theory, B2. Daniel Minoli, Sufficient Forms For Generalized Perfect Numbers, Ann. Fac. Sciences, Univ. Nation. Zaire, Section Mathem; Vol. 4, No. 2, Dec 1978, pp. 277-302. Daniel Minoli, New Results For Hyperperfect Numbers, Abstracts American Math. Soc., October 1980, Issue 6, Vol. 1, p. 561. Daniel Minoli, Voice Over MPLS, McGraw-Hill, 2002, New York, NY, see pp. 112-134. Daniel Minoli and Robert Bear, Hyperperfect Numbers, PME Journal, Fall 1975, pp. 153-157. Daniel Minoli and W. Nakamine, Mersenne Numbers Rooted On 3 For Number Theoretic Transforms, 1980 IEEE International Conf. on Acoust., Speech and Signal Processing. J. Roberts, Lure of the Integers, Math. Assoc. America, 1992, p. 177. N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence). LINKS Antal Bege, Kinga Fogarasi, Generalized perfect numbers, Acta Univ. Sapientiae, Math., 1 (2009), 73-82. J. S. McCranie, A study of hyperperfect numbers, J. Int. Seqs. Vol. 3 (2000) #P00.1.3. Daniel Minoli, Issues In Non-Linear Hyperperfect Numbers, Mathematics of Computation, Vol. 34, No. 150, April 1980, pp. 639-645. Daniel Minoli, Structural Issues For Hyperperfect Numbers, Fibonacci Quarterly, Feb. 1981, Vol. 19, No. 1, pp. 6-14. Eric Weisstein's World of Mathematics, Hyperperfect Number MATHEMATICA t={}; Do[p=3^n-2; If[PrimeQ[p], q=3^(n-1)*p; AppendTo[t, q]], {n, 100}]; t (* Jayanta Basu, May 02 2013 *) PROG (PARI) is(n)=n==2*(sigma(n)-n-1) + 1 \\ Charles R Greathouse IV, May 01 2013 CROSSREFS Cf. A000396, A028499, A028500, A028501, A028502, A034916, A220290. Sequence in context: A304643 A153848 A221771 * A219104 A219983 A119099 Adjacent sequences:  A007590 A007591 A007592 * A007594 A007595 A007596 KEYWORD nonn,hard,more AUTHOR EXTENSIONS More terms from Jud McCranie, Dec 16 1999 STATUS approved

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Last modified August 22 11:34 EDT 2019. Contains 326176 sequences. (Running on oeis4.)