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%I M4186 N1744
%S 6,28,496,8128,33550336,8589869056,137438691328,2305843008139952128,
%T 2658455991569831744654692615953842176,
%U 191561942608236107294793378084303638130997321548169216
%N Perfect numbers n: n is equal to the sum of the proper divisors of n.
%C 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).
%C For number of divisors of a(n) see A061645(n). Number of digits in a(n) is A061193(n). - _Lekraj Beedassy_, Jun 04 2004
%C 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 Mersenne primes M=2^p - 1, for odd p, are of form 6*t+1. Thus perfect numbers N, being M-th triangular, have 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
%C The earliest recorded mention of this sequence is in Euclid's Elements, IX 36, about 300 BC. - _Artur Jasinski_, Jan 25 2006
%C 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
%C Theorem (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. It follows that every even perfect number is also a triangular number. - _Mohammad K. Azarian_, Apr 16 2008
%C Triangular numbers A000217 whose indices are Mersenne primes A000668, assuming there are no odd perfect numbers. 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
%C Hexagonal numbers A000384 whose indices are superperfect numbers A019279, assuming there are no odd perfect numbers and no odd superperfect numbers. [From _Omar E. Pol_, Aug 17 2008]
%C 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. [From _Omar E. Pol_, Aug 30 2008]
%C Contribution from _Reikku Kulon_, Oct 14 2008: (Start)
%C A144912(2, a(n)) = 1;
%C A144912(4, a(n)) = -1 for n > 1;
%C A144912(8, a(n)) = 5 or -5 for all n except 2;
%C A144912(16, a(n)) = -4 or -13 for n > 1. (End)
%C Multiply-perfect numbers A007691 whose indices are the numbers A153800, assuming there are no odd perfect numbers. [From _Omar E. Pol_, Jan 14 2009]
%C If a(n) is even, then 2*a(n) is in A181595. [From _Vladimir Shevelev_, Nov 07 2010]
%C Except for 6, all even terms are of the form 30*k - 2 or 45*k + 1. [_Arkadiusz Wesolowski_, Mar 11 2012]
%D T. M. Apostol, Introduction to Analytic Number Theory, Springer-Verlag, 1976, page 4.
%D A. H. Beiler, Recreations in the Theory of Numbers, Dover, NY, 1964, p. 19.
%D S. Bezuszka, Perfect Numbers, (Booklet 3, Motivated Math. Project Activities) Boston College Press, Chestnut Hill MA 1980.
%D Euclid, Elements, Book IX, Section 36, about 300 BC.
%D G. H. Hardy and E. M. Wright, An Introduction to the Theory of Numbers. 3rd ed., Oxford Univ. Press, 1954, p. 239.
%D T. Koshy, "The Ends Of A Mersenne Prime And An Even Perfect Number", Journal of Recreational Mathematics, pp. 196-202 Baywood NY 1998.
%D 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)
%D J. Sandor, Handbook of Number Theory, II, Springer Verlag, 2004.
%D N. J. A. Sloane, A Handbook of Integer Sequences, Academic Press, 1973 (includes this sequence).
%D N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
%D I. Stewart, L'univers des nombres, "Diviser Pour Regner", Chapter 14, pp. 74-81 Belin-Pour La Science, Paris 2000.
%D H. S. Uhler, On the 16th and 17th perfect numbers, Scripta Math. 19 (1953), 128-131.
%D D. Wells, The Penguin Dictionary of Curious and Interesting Numbers, pp. 107-110 Penguin Books 1987.
%H David Wasserman, <a href="/A000396/b000396.txt">Table of n, a(n) for n = 1..14</a>
%H Anonymous, <a href="http://www-maths.swan.ac.uk/pgrads/bb/project/node3.html">Perfect Numbers</a> [broken link]
%H Anonymous, <a href="http://www-maths.swan.ac.uk/pgrads/bb/project/node43.html">Timetable of discovery of perfect numbers</a> [broken link]
%H R. P. Brent & G. L. Cohen, <a href="http://wwwmaths.anu.edu.au/~brent/pub/pub100.html">A new lower bound for odd perfect numbers</a>
%H R. P. Brent, G. L. Cohen & H. J. J. te Riele, <a href="http://wwwmaths.anu.edu.au/~brent/pub/pub106.html">A new approach to lower bounds for odd perfect numbers</a>
%H R. P. Brent, G. L. Cohen & H. J. J. te Riele, <a href="http://db.cwi.nl/rapporten/abstract.php?abstractnr=1802">Improved Techniques For Lower Bounds For Odd Perfect Numbers</a>
%H J. Britton, <a href="http://britton.disted.camosun.bc.ca/perfect/jbperfect.htm">Perfect Number Analyser</a>
%H C. K. Caldwell, <a href="http://primes.utm.edu/glossary/page.php?sort=PerfectNumber">Perfect number</a>
%H C. K. Caldwell, <a href="http://www.utm.edu/research/primes/mersenne/index.html">Mersenne Primes, etc.</a>
%H C. K. Caldwell, <a href="http://www.utm.edu/research/primes/notes/proofs/Theorem3.html">Iterated sums of the digits of a perfect number converge to 1</a>
%H J. W. Gaberdiel, <a href="http://math.arizona.edu/~ura/001/gaberdiel.jw/">A Study of Perfect Numbers and Related Topics</a>
%H T. Goto & Y. Ohno, <a href="http://www.ma.noda.tus.ac.jp/u/tg/perfect.html">Largest prime factor of an odd perfect number</a>
%H K. G. Hare, <a href="http://arXiv.org/abs/math.NT/0501070">New techniques for bounds on the total number of Prime Factors of an Odd Perfect Number</a>
%H D. & C. Hazzlewood, <a href="http://www.math.swt.edu/~haz/prob_sets/notes/node13.html">Perfect Numbers</a> <a href="/A000396/a000396_h.html">[Cached copy]</a>
%H C.-E. Jean, "Recreomath" Online Dictionary, <a href="http://www.recreomath.qc.ca/dict_parfait_nombre.htm">Nombre parfait</a>
%H T. Leinster, <a href="http://arXiv.org/abs/math.GR/0104012">Perfect numbers and groups</a>, arXiv:math.GR/0104012
%H T. Masiwa, T. Shonhiwa & G. Hitchcock, <a href="http://uzweb.uz.ac.zw/science/maths/zimaths/51/perfect.htm">Perfect Numbers & Mersenne Primes</a>
%H Mathforum, <a href="http://mathforum.org/dr.math/faq/faq.perfect.html">Perfect Numbers</a>
%H Mathforum, <a href="http://mathforum.org/library/drmath/view/51516.html">List of Perfect Numbers</a>
%H J. S. McCranie, <a href="http://www.cs.uwaterloo.ca/journals/JIS/index.html">A study of hyperperfect numbers, J. Int. Seqs. Vol. 3 (2000) #P00.1.3</a>
%H G. P. Michon, <a href="http://www.numericana.com/answer/numbers.htm#perfect">Perfect Numbers, Mersenne Primes</a>
%H D. Moews, <a href="http://djm.cc/amicable.html">Perfect, amicable and sociable numbers</a>
%H P. P. Nielsen, <a href="http://fr.arXiv.org/abs/math.NT/0602485">Odd Perfect Numbers Have At Least Nine Distinct Prime Factors</a>
%H Walter Nissen, <a href="http://upforthecount.com/math/abundance.html">Abundancy : Some Resources </a>
%H J. J. O'Connor & E. F. Robertson, <a href="http://www-groups.dcs.st-and.ac.uk/~history/HistTopics/Perfect_numbers.html">Perfect Numbers</a>
%H J. O. M. Pedersen, <a href="http://amicable.homepage.dk/perfect.htm">Perfect numbers</a>
%H J. O. M. Pedersen, <a href="http://amicable.homepage.dk/tables.htm">Tables of Aliquot Cycles</a>
%H I. Peterson, <a href="http://www.maa.org/mathland/mathtrek_5_18_98.html">Cubes of Perfection</a>
%H O. E. Pol, <a href="http://www.polprimos.com">Determinacion geometrica de los numeros primos y perfectos</a>.
%H D. Romagnoli, <a href="http://www.mistretta.eu/PDF/I%20numeri%20perfetti.pdf">Perfect Numbers (Text in Italian)</a> [From _Lekraj Beedassy_, Jun 26 2009]
%H D. Scheffler, R. Ondrejka, <a href="http://dx.doi.org/10.1090/S0025-5718-1960-0112239-6">The numerical evaluation of the eighteenth perfect number</a>, Math. Comp. 14 (70) (1960) 199-200
%H K. Schneider, PlanetMath.org, <a href="http://planetmath.org/encyclopedia/PerfectNumber.html">perfect number</a>
%H G. Villemin's Almanach of Numbers, <a href="http://villemin.gerard.free.fr/Wwwgvmm/Decompos/SixNbPf.htm">Nombres Parfaits</a>
%H J. Voight, <a href="http://magma.maths.usyd.edu.au/~voight/notes/perfelem.pdf">Perfect Numbers:An Elementary Introduction</a>
%H Eric Weisstein's World of Mathematics, <a href="http://mathworld.wolfram.com/PerfectNumber.html">Perfect Number</a>
%H Eric Weisstein's World of Mathematics, <a href="http://mathworld.wolfram.com/OddPerfectNumber.html">Odd Perfect Number</a>
%H Eric Weisstein's World of Mathematics, <a href="http://mathworld.wolfram.com/MultiperfectNumber.html">Multiperfect Number</a>
%H Eric Weisstein's World of Mathematics, <a href="http://mathworld.wolfram.com/HyperperfectNumber.html">Hyperperfect Number</a>
%H Eric Weisstein's World of Mathematics, <a href="http://mathworld.wolfram.com/Abundance.html">Abundance</a>
%H Wikipedia, <a href="http://www.wikipedia.org/wiki/Perfect_number">Perfect number</a>
%H T. Yamada, <a href="http://arXiv.org/abs/math.NT/0511410">On the divisibility of odd perfect numbers by a high power of a prime</a>
%H <a href="/index/Cor#core">Index entries for "core" sequences</a>
%F 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.
%F Numbers n such that sum(d|n, 1/d)=2 - _Benoit Cloitre_, Apr 07 2002
%F 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
%F a(n) = 2^A133033(n) - 2^A090748(n), assuming there are no odd perfect numbers. - _Omar E. Pol_, Feb 28 2008
%F a(n) = A000668(n)*(A000668(n)+1)/2, assuming there are no odd perfect numbers. - _Omar E. Pol_, Apr 23 2008
%F a(n) = A000217(A000668(n)), assuming there are no odd perfect numbers. - _Omar E. Pol_, May 09 2008
%F a(n) = Sum of first A000668(n) positive integers, assuming there are no odd perfect numbers. - _Omar E. Pol_, May 09 2008
%F 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. [From _Omar E. Pol_, Aug 17 2008]
%F a(n) = A006516(A000043(n)), assuming there are no odd perfect numbers. [From _Omar E. Pol_, Aug 30 2008]
%F 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. [From _Omar E. Pol_, Jan 09 2009]
%F a(n) = A007691(A153800(n)), assuming there are no odd perfect numbers. [From _Omar E. Pol_, Jan 14 2009]
%F 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). [From _Lekraj Beedassy_, May 02 2009]
%F a(n) = A060286(A016027(n)), assuming there are no odd perfect numbers. - _Omar E. Pol_, Dec 13 2012
%e 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.
%p ZL:=[]: for p from 1 to 101 do if (isprime(p) and isprime(2^p-1)) then ZL:=[op(ZL),2^(p-1)*(2^p-1)]; fi; od; print(ZL); - Zerinvary Lajos (zerinvarylajos(AT)yahoo.com), Feb 05 2008
%t A000396 = (# (# + 1))/2 & /@ Select[2^Range[100] - 1, PrimeQ] (* from _Harvey P. Dale_, Mar 06 2002, Apr 06 2011 *)
%o (PARI) isA000396(n) = (sigma(n) == 2*n)
%o forprime(p=1,90,if(isprime(2^p-1),print(2^(p-1)*(2^p-1)))) \\ _Michael B. Porter_, Nov 03 2009
%o (Haskell)
%o a000396 n = a000396_list !! (n-1)
%o a000396_list = [x | x <- [1..], a000203 x == 2 * x]
%o -- _Reinhard Zumkeller_, Jan 20 2012
%Y See A000043 for the current state of knowledge about Mersenne primes.
%Y Cf. A007539, A005820, A027687, A046060, A046061, A000668, A090748, A133033, A000217, A000384, A019279, A061652, A006516, A144912, A007691, A153800, A007593, A220290, A028499-A028502, A034916.
%K nonn,nice,core
%O 1,1
%A _N. J. A. Sloane_.
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