This site is supported by donations to The OEIS Foundation. Hints (Greetings from The On-Line Encyclopedia of Integer Sequences!)
 A000396 Perfect numbers n: n is equal to the sum of the proper divisors of n. (Formerly M4186 N1744) 475
 6, 28, 496, 8128, 33550336, 8589869056, 137438691328, 2305843008139952128, 2658455991569831744654692615953842176, 191561942608236107294793378084303638130997321548169216 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS A number n is abundant if sigma(n) > 2n (cf. A005101), perfect if sigma(n) = 2n (this entry), deficient if sigma(n) < 2n (cf. A005100), where sigma(n) is the sum of the divisors of n (A000203). The numbers 2^(p-1)(2^p - 1) are perfect, where p is a prime such that 2^p - 1 is also prime (for the list of p's see A000043). There are no other even perfect numbers and it is believed that there are no odd perfect numbers. Numbers n such that Sum_{d|n} 1/d = 2. - Benoit Cloitre, Apr 07 2002 For number of divisors of a(n) see A061645(n). Number of digits in a(n) is A061193(n). - Lekraj Beedassy, Jun 04 2004 All entries other than the first have digital root 1 (since 4^2 == 4 (mod 6), we have, by induction, 4^k == 4 (mod 6), or 2*2^(2*k) = 8 == 2 (mod 6) implying that Mersenne primes M = 2^p - 1, for odd p, are of the form 6*t+1. Thus perfect numbers N, being M-th triangular, have the form (6*t+1)*(3*t+1), whence the property N mod 9 = 1 for all N after the first. - Lekraj Beedassy, Aug 21 2004 The earliest recorded mention of this sequence is in Euclid's Elements, IX 36, about 300 BC. - Artur Jasinski, Jan 25 2006 The number of divisors of a(n) that are powers of 2 is equal to A000043(n), assuming there are no odd perfect numbers. The number of divisors of a(n) that are multiples of n-th Mersenne prime A000668(n) is also equal to A000043(n), again assuming there are no odd perfect numbers. - Omar E. Pol, Feb 28 2008 Theorem (Euclid, Euler). An even number n is a perfect number if and only if n = 2^(k-1)*(2^k-1), where 2^k-1 is prime. Euler's idea came from Euclid's Proposition 36 of Book IX (see Weil). It follows that every even perfect number is also a triangular number. - Mohammad K. Azarian, Apr 16 2008 Triangular numbers (also generalized hexagonal numbers) A000217 whose indices are Mersenne primes A000668, assuming there are no odd perfect numbers. - Omar E. Pol, May 09 2008, Sep 15 2013 Sum of first m positive integers, where m is the n-th Mersenne prime A000668(n), assuming there are no odd perfect numbers. - Omar E. Pol, May 09 2008 Hexagonal numbers A000384 whose indices are superperfect numbers A019279, assuming there are no odd perfect numbers and no odd superperfect numbers. - Omar E. Pol, Aug 17 2008 It appears that this sequence is equal to the numbers A006516 whose indices are the prime numbers A000043, assuming there are no odd perfect numbers. - Omar E. Pol, Aug 30 2008 From Reikku Kulon, Oct 14 2008: (Start) A144912(2, a(n)) = 1; A144912(4, a(n)) = -1 for n > 1; A144912(8, a(n)) = 5 or -5 for all n except 2; A144912(16, a(n)) = -4 or -13 for n > 1. (End) Multiply-perfect numbers A007691 whose indices are the numbers A153800, assuming there are no odd perfect numbers. - Omar E. Pol, Jan 14 2009 If a(n) is even, then 2*a(n) is in A181595. - Vladimir Shevelev, Nov 07 2010 Except for 6, all even terms are of the form 30*k - 2 or 45*k + 1. - Arkadiusz Wesolowski, Mar 11 2012 a(4) = A229381(1) = 8128 is the "Simpsons's perfect number". - Jonathan Sondow, Jan 02 2015 Theorem (Farideh Firoozbakht): If m is an integer and both p and p^k-m-1 are prime numbers then x = p^(k-1)*(p^k-m-1) is a solution to the equation sigma(x) = (p*x+m)/(p-1). For example, if we take m=0 and p=2 we get Euclid's result about perfect numbers. - Farideh Firoozbakht, Mar 01 2015 The cototient of the even perfect numbers is square; in particular, if 2^p - 1 is a Mersenne prime, cototient(2^(p-1) * (2^p - 1)) = (2^(p-1))^2. So, this sequence is a subsequence of A063752. - Bernard Schott, Jan 11 2019 REFERENCES T. M. Apostol, Introduction to Analytic Number Theory, Springer-Verlag, 1976, page 4. A. H. Beiler, Recreations in the Theory of Numbers, Dover, NY, 1964, p. 19. S. Bezuszka, Perfect Numbers, (Booklet 3, Motivated Math. Project Activities) Boston College Press, Chestnut Hill MA 1980. Euclid, Elements, Book IX, Section 36, about 300 BC. G. H. Hardy and E. M. Wright, An Introduction to the Theory of Numbers. 3rd ed., Oxford Univ. Press, 1954, p. 239. T. Koshy, "The Ends Of A Mersenne Prime And An Even Perfect Number", Journal of Recreational Mathematics, pp. 196-202 Baywood NY 1998. Joseph S. Madachy: Madachy's Mathematical Recreations. New York: Dover Publications, Inc., 1979, p. 149 (First publ. by Charles Scribner's Sons, New York, 1966, under the title: Mathematics on Vacation) J. Sandor, Handbook of Number Theory, II, Springer Verlag, 2004. N. J. A. Sloane, A Handbook of Integer Sequences, Academic Press, 1973 (includes this sequence). N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence). I. Stewart, L'univers des nombres, "Diviser Pour Régner", Chapter 14, pp. 74-81 Belin-Pour La Science, Paris 2000. H. S. Uhler, On the 16th and 17th perfect numbers, Scripta Math. 19 (1953), 128-131. André Weil, Number Theory, An approach through history, From Hammurapi to Legendre, Birkhäuser, 1984, p 6. D. Wells, The Penguin Dictionary of Curious and Interesting Numbers, pp. 107-110 Penguin Books 1987. LINKS E-Hern Lee, Table of n, a(n) for n = 1..15 (terms 1-14 from David Wasserman) Abiodun E. Adeyemi, A Study of @-numbers, arXiv:1906.05798 [math.NT], 2019. Anonymous, Perfect Numbers [broken link] Anonymous, Timetable of discovery of perfect numbers [broken link] Antal Bege, Kinga Fogarasi, Generalized perfect numbers, arXiv:1008.0155 [math.NT], 2010. R. P. Brent & G. L. Cohen, A new lower bound for odd perfect numbers R. P. Brent, G. L. Cohen & H. J. J. te Riele, A new approach to lower bounds for odd perfect numbers R. P. Brent, G. L. Cohen & H. J. J. te Riele, Improved Techniques For Lower Bounds For Odd Perfect Numbers J. Britton, Perfect Number Analyser C. K. Caldwell, Perfect number C. K. Caldwell, Mersenne Primes, etc. C. K. Caldwell, Iterated sums of the digits of a perfect number converge to 1 J. A. B. Dris, The Abundancy Index of Divisors of Odd Perfect Numbers, J. Int. Seq. 15 (2012) # 12.4.4 Jason Earls, The Smarandache sum of composites between factors function, in Smarandache Notions Journal (2004), Vol. 14.1, page 243. Bakir Farhi, On the representation of an even perfect number as the sum of a limited number of cubes, arXiv:1504.07322 [math.NT], 2015. Steven Finch, Amicable Pairs and Aliquot Sequences, 2013. [Cached copy, with permission of the author] F. Firoozbakht, M. F. Hasler, Variations on Euclid's formula for Perfect Numbers, JIS 13 (2010) #10.3.1. J. W. Gaberdiel, A Study of Perfect Numbers and Related Topics T. Goto & Y. Ohno, Largest prime factor of an odd perfect number K. G. Hare, New techniques for bounds on the total number of Prime Factors of an Odd Perfect Number, arXiv:math/0501070 [math.NT], 2005-2006. A. Hoque, H. Kalita, Generalized perfect numbers connected with arithmetic functions, Math. Sci. Lett. 3, No. 3, 249-253 (2014). C.-E. Jean, "Recreomath" Online Dictionary, Nombre parfait Christian Kassel and Christophe Reutenauer, The zeta function of the Hilbert scheme of n points on a two-dimensional torus, arXiv:1505.07229v3 [math.AG], 2015. [A later version of this paper has a different title and different contents, and the number-theoretical part of the paper was moved to the publication below.] Christian Kassel and Christophe Reutenauer, Complete determination of the zeta function of the Hilbert scheme of n points on a two-dimensional torus, arXiv:1610.07793 [math.NT], 2016. T. Leinster, Perfect numbers and groups, arXiv:math/0104012 [math.GR], 2001. A. V. Lelechenko, The Quest for the Generalized Perfect Numbers, in Theoretical and Applied Aspects of Cybernetics, TAAC 2014, Kiev. Daniel Lustig, The algebraic independence of the sum of divisors functions, Journal of Number Theory, Volume 130, Issue 11, November 2010, Pages 2628-2633. T. Masiwa, T. Shonhiwa & G. Hitchcock, Perfect Numbers & Mersenne Primes Mathforum, Perfect Numbers Mathforum, List of Perfect Numbers J. S. McCranie, A study of hyperperfect numbers, J. Int. Seqs. Vol. 3 (2000) #P00.1.3. G. P. Michon, Perfect Numbers, Mersenne Primes D. Moews, Perfect, amicable and sociable numbers P. P. Nielsen, Odd Perfect Numbers Have At Least Nine Distinct Prime Factors, arXiv:math/0602485 [math.NT], 2006. Walter Nissen, Abundancy : Some Resources J. J. O'Connor & E. F. Robertson, Perfect Numbers J. O. M. Pedersen, Perfect numbers [Via Internet Archive Wayback-Machine] J. O. M. Pedersen, Tables of Aliquot Cycles [Broken link] J. O. M. Pedersen, Tables of Aliquot Cycles [Via Internet Archive Wayback-Machine] J. O. M. Pedersen, Tables of Aliquot Cycles [Cached copy, pdf file only] I. Peterson, Cubes of Perfection P. Pollack, Quasi-Amicable Numbers are Rare, J. Int. Seq. 14 (2011) # 11.5.2 D. Romagnoli, Perfect Numbers (Text in Italian) [From Lekraj Beedassy, Jun 26 2009] D. Scheffler, R. Ondrejka, The numerical evaluation of the eighteenth perfect number, Math. Comp. 14 (70) (1960) 199-200 K. Schneider, PlanetMath.org, perfect number J. Sondow and K. MacMillan, Primary pseudoperfect numbers, arithmetic progressions, and the Erdos-Moser equation, Amer. Math. Monthly, 124 (2017) 232-240; arXiv:math/1812.06566 [math.NT], 2018. G. Villemin's Almanach of Numbers, Nombres Parfaits J. Voight, Perfect Numbers:An Elementary Introduction Eric Weisstein's World of Mathematics, Perfect Number Eric Weisstein's World of Mathematics, Odd Perfect Number Eric Weisstein's World of Mathematics, Multiperfect Number Eric Weisstein's World of Mathematics, Hyperperfect Number Eric Weisstein's World of Mathematics, Abundance Wikipedia, Perfect number T. Yamada, On the divisibility of odd perfect numbers by a high power of a prime, arXiv:math/0511410 [math.NT], 2005-2007. FORMULA The perfect number N = {2^(p-1)}*(2^p - 1) is also multiplicatively p-perfect (i.e., A007955(N)=N^p), since tau(N) = 2p. - Lekraj Beedassy, Sep 21 2004 a(n) = 2^A133033(n) - 2^A090748(n), assuming there are no odd perfect numbers. - Omar E. Pol, Feb 28 2008 a(n) = A000668(n)*(A000668(n)+1)/2, assuming there are no odd perfect numbers. - Omar E. Pol, Apr 23 2008 a(n) = A000217(A000668(n)), assuming there are no odd perfect numbers. - Omar E. Pol, May 09 2008 a(n) = Sum of first A000668(n) positive integers, assuming there are no odd perfect numbers. - Omar E. Pol, May 09 2008 a(n) = A000384(A019279(n)), assuming there are no odd perfect numbers and no odd superperfect numbers. a(n)= A000384(A061652(n)), assuming there are no odd perfect numbers. - Omar E. Pol, Aug 17 2008 a(n) = A006516(A000043(n)), assuming there are no odd perfect numbers. - Omar E. Pol, Aug 30 2008 a(n) = A019279(n)*A000668(n), assuming there are no odd perfect numbers and odd superperfect numbers. a(n) = A061652(n)*A000668(n), assuming there are no odd perfect numbers. - Omar E. Pol, Jan 09 2009 a(n) = A007691(A153800(n)), assuming there are no odd perfect numbers. - Omar E. Pol, Jan 14 2009 Even perfect numbers N = K*A000203(K), where K = A019279(n) = 2^(p-1), A000203(A019279(n)) = A000668(n) = 2^p - 1 = M(p), p = A000043(n). - Lekraj Beedassy, May 02 2009 a(n) = A060286(A016027(n)), assuming there are no odd perfect numbers. - Omar E. Pol, Dec 13 2012 For n >= 2, a(n) = Sum_{k=1..A065549(n)} (2*k-1)^3, assuming there are no odd perfect numbers. - Derek Orr, Sep 28 2013 a(n) = A275496(2^((A000043(n) - 1)/2)) - 2^A000043(n), assuming there are no odd perfect numbers. - Daniel Poveda Parrilla, Aug 16 2016 a(n) = A156552(A324201(n)), assuming there are no odd perfect numbers. - Antti Karttunen, Mar 28 2019 EXAMPLE 6 is perfect because 6 = 1+2+3, the sum of all divisors of 6 less than 6; 28 is perfect because 28 = 1+2+4+7+14. MATHEMATICA Select[Range, DivisorSigma[1, #]== 2*# &] (* G. C. Greubel, Oct 03 2017 *) PerfectNumber[Range] (* Requires Mathematica version 10 or later *) (* Harvey P. Dale, Dec 10 2018 *) PROG (PARI) isA000396(n) = (sigma(n) == 2*n); (Haskell) a000396 n = a000396_list !! (n-1) a000396_list = [x | x <- [1..], a000203 x == 2 * x] -- Reinhard Zumkeller, Jan 20 2012 CROSSREFS See A000043 for the current state of knowledge about Mersenne primes. Cf. A007539, A005820, A027687, A046060, A046061, A000668, A090748, A133033, A000217, A000384, A019279, A061652, A006516, A144912, A007691, A153800, A007593, A220290, A028499-A028502, A034916, A065549, A275496, A063752, A156552, A324201. Sequence in context: A325654 A201186 A060286 * A152953 A066239 A097464 Adjacent sequences:  A000393 A000394 A000395 * A000397 A000398 A000399 KEYWORD nonn,nice,core AUTHOR STATUS approved

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recent
The OEIS Community | Maintained by The OEIS Foundation Inc.

Last modified August 18 10:55 EDT 2019. Contains 326078 sequences. (Running on oeis4.)