Perfect number

For the 2012 film, see Perfect Number (film).
Demonstration, with Cuisenaire rods, of the perfection of the number 6

In number theory, a perfect number is a positive integer that is equal to the sum of its proper positive divisors, that is, the sum of its positive divisors excluding the number itself (also known as its aliquot sum). Equivalently, a perfect number is a number that is half the sum of all of its positive divisors (including itself) i.e. σ1(n) = 2n.

This definition is ancient, appearing as early as Euclid's Elements (VII.22) where it is called τέλειος ἀριθμός (perfect, ideal, or complete number). Euclid also proved a formation rule (IX.36) whereby q(q+1)/2 is an even perfect number whenever q is what is now called a Mersenne prime—a prime of the form 2^p -1 for prime p. Much later, Euler proved that all even perfect numbers are of this form.[1] This is known as the Euclid–Euler theorem.

It is not known whether there are any odd perfect numbers, nor whether infinitely many perfect numbers exist.

Examples

The first perfect number is 6, because 1, 2, and 3 are its proper positive divisors, and 1 + 2 + 3 = 6. Equivalently, the number 6 is equal to half the sum of all its positive divisors: ( 1 + 2 + 3 + 6 ) / 2 = 6. The next perfect number is 28 = 1 + 2 + 4 + 7 + 14. This is followed by the perfect numbers 496 and 8128 (sequence A000396 in OEIS).

History

These first four perfect numbers were the only ones known to early Greek mathematics, and the mathematician Nicomachus had noted 8128 as early as 100 AD.[2] Philo of Alexandria in his first-century book "On the creation" mentions perfect numbers, claiming that the world was created in 6 days and the moon orbits in 28 days because 6 and 28 are perfect. Philo is followed by Origen,[3] and by Didymus the Blind, who adds the observation that there are only four perfect numbers that are less than 10,000. (Commentary on Genesis 1. 14-19).[4] St Augustine defines perfect numbers in City of God (Part XI, Chapter 30) in the early 5th century AD, repeating the claim that God created the world in 6 days because 6 is the smallest perfect number. The Egyptian mathematician Ismail ibn Fallūs (1194–1252) mentioned the next three perfect numbers (33,550,336, 8,589,869,056 and 137,438,691,328) and listed a few more which are now known to be incorrect.[5] In a manuscript written between 1456 and 1461, an unknown mathematician recorded the earliest European reference to a fifth perfect number, with 33,550,336 being correctly identified for the first time.[6][7] In 1588, the Italian mathematician Pietro Cataldi also identified the sixth (8,589,869,056) and the seventh (137,438,691,328) perfect numbers, and also proved that every perfect number obtained from Euclid's rule ends with a 6 or an 8.[8][9][10]

Even perfect numbers

Unsolved problem in mathematics:
Are there infinitely many perfect numbers?
(more unsolved problems in mathematics)

Euclid proved that 2p−1(2p  1) is an even perfect number whenever 2p  1 is prime (Euclid, Prop. IX.36).

For example, the first four perfect numbers are generated by the formula 2p−1(2p  1), with p a prime number, as follows:

for p = 2:   21(22  1) = 6
for p = 3:   22(23  1) = 28
for p = 5:   24(25  1) = 496
for p = 7:   26(27  1) = 8128.

Prime numbers of the form 2p  1 are known as Mersenne primes, after the seventeenth-century monk Marin Mersenne, who studied number theory and perfect numbers. For 2p  1 to be prime, it is necessary that p itself be prime. However, not all numbers of the form 2p  1 with a prime p are prime; for example, 211  1 = 2047 = 23 × 89 is not a prime number.[11] In fact, Mersenne primes are very rare—of the 9,592 prime numbers p less than 100,000,[12] 2p  1 is prime for only 28 of them.

Over a millennium after Euclid, Ibn al-Haytham (Alhazen) circa 1000 AD conjectured that every even perfect number is of the form 2p−1(2p  1) where 2p  1 is prime, but he was not able to prove this result.[13] It was not until the 18th century that Leonhard Euler proved that the formula 2p−1(2p  1) will yield all the even perfect numbers. Thus, there is a one-to-one correspondence between even perfect numbers and Mersenne primes; each Mersenne prime generates one even perfect number, and vice versa. This result is often referred to as the Euclid–Euler theorem. As of January 2016, 49 Mersenne primes are known,[14] and therefore 49 even perfect numbers (the largest of which is 274207280 × (274207281  1) with 44,677,235 digits).

An exhaustive search by the GIMPS distributed computing project has shown that the first 44 even perfect numbers are 2p−1(2p  1) for

p = 2, 3, 5, 7, 13, 17, 19, 31, 61, 89, 107, 127, 521, 607, 1279, 2203, 2281, 3217, 4253, 4423, 9689, 9941, 11213, 19937, 21701, 23209, 44497, 86243, 110503, 132049, 216091, 756839, 859433, 1257787, 1398269, 2976221, 3021377, 6972593, 13466917, 20996011, 24036583, 25964951, 30402457, and 32582657 (sequence A000043 in OEIS).[15]

Five higher perfect numbers have also been discovered, namely those for which p = 37156667, 42643801, 43112609, 57885161, and 74207281, though there may be others within this range. It is not known whether there are infinitely many perfect numbers, nor whether there are infinitely many Mersenne primes.

As well as having the form 2p−1(2p  1), each even perfect number is the (2p  1)th triangular number (and hence equal to the sum of the integers from 1 to 2p  1) and the 2p−1th hexagonal number. Furthermore, each even perfect number except for 6 is the ((2p + 1)/3)th centered nonagonal number and is equal to the sum of the first 2(p−1)/2 odd cubes:


\begin{align}
6 & = 2^1(2^2-1) & & = 1+2+3, \\[8pt]
28 & = 2^2(2^3-1) & & = 1+2+3+4+5+6+7 = 1^3+3^3, \\[8pt]
496 & = 2^4(2^5-1) & & = 1+2+3+\cdots+29+30+31 \\
& & & = 1^3+3^3+5^3+7^3, \\[8pt]
8128 & = 2^6(2^7-1) & & = 1+2+3+\cdots+125+126+127 \\
& & & = 1^3+3^3+5^3+7^3+9^3+11^3+13^3+15^3, \\[8pt]
33550336 & = 2^{12}(2^{13}-1) & & = 1+2+3+\cdots+8189+8190+8191 \\
& & & = 1^3+3^3+5^3+\cdots+123^3+125^3+127^3.
\end{align}

Even perfect numbers (except 6) are of the form

T_{2^p - 1} = 1 + \frac{(2^p-2) \times (2^p+1)}{2} = 1 + 9 \times T_{(2^p - 2)/3}

with each resulting triangular number (after subtracting 1 from the perfect number and dividing the result by 9) ending in 3 or 5, the sequence starting with 3, 55, 903, 3727815, ....[16] This can be reformulated as follows: adding the digits of any even perfect number (except 6), then adding the digits of the resulting number, and repeating this process until a single digit (called the digital root) is obtained, always produces the number 1. For example, the digital root of 8128 is 1, because 8 + 1 + 2 + 8 = 19, 1 + 9 = 10, and 1 + 0 = 1. This works with all perfect numbers 2p−1(2p  1) with odd prime p and, in fact, with all numbers of the form 2m−1(2m  1) for odd integer (not necessarily prime) m.

Owing to their form, 2p−1(2p  1), every even perfect number is represented in binary as p ones followed by p  1  zeros:

610 = 1102
2810 = 111002
49610 = 1111100002
812810 = 11111110000002
3355033610 = 11111111111110000000000002.

Thus every even perfect number is a pernicious number.

Note that every even perfect number is also a practical number (c.f. Related concepts).

Odd perfect numbers

Unsolved problem in mathematics:
Are there any odd perfect numbers?
(more unsolved problems in mathematics)

It is unknown whether there is any odd perfect number, though various results have been obtained. In 1496, Jacques Lefèvre stated that Euclid's rule gives all perfect numbers,[17] thus implying that no odd perfect number exists. More recently, Carl Pomerance has presented a heuristic argument suggesting that indeed no odd perfect number should exist.[18] All perfect numbers are also Ore's harmonic numbers, and it has been conjectured as well that there are no odd Ore's harmonic numbers other than 1.

Any odd perfect number N must satisfy the following conditions:

N=q^{\alpha} p_1^{2e_1} \cdots p_k^{2e_k},
where:
  • q, p1, ..., pk are distinct primes (Euler).
  • q α ≡ 1 (mod 4) (Euler).
  • The smallest prime factor of N is less than (2k + 8) / 3.[22]
  • Either qα > 1062, or p j2ej  > 1062 for some j.[19]
  • N < 24k+1.[23]

In 1888, Sylvester stated:[29]

...a prolonged meditation on the subject has satisfied me that the existence of any one such [odd perfect number] — its escape, so to say, from the complex web of conditions which hem it in on all sides — would be little short of a miracle.

Euler stated: "Whether (...) there are any odd perfect numbers is a most difficult question".[30]

Minor results

All even perfect numbers have a very precise form; odd perfect numbers either do not exist or are rare. There are a number of results on perfect numbers that are actually quite easy to prove but nevertheless superficially impressive; some of them also come under Richard Guy's strong law of small numbers:

Related concepts

The sum of proper divisors gives various other kinds of numbers. Numbers where the sum is less than the number itself are called deficient, and where it is greater than the number, abundant. These terms, together with perfect itself, come from Greek numerology. A pair of numbers which are the sum of each other's proper divisors are called amicable, and larger cycles of numbers are called sociable. A positive integer such that every smaller positive integer is a sum of distinct divisors of it is a practical number.

By definition, a perfect number is a fixed point of the restricted divisor function s(n) = σ(n) − n, and the aliquot sequence associated with a perfect number is a constant sequence. All perfect numbers are also \mathcal{S}-perfect numbers, or Granville numbers.

A semiperfect number is a natural number that is equal to the sum of all or some of its proper divisors. A semiperfect number that is equal to the sum of all its proper divisors is a perfect number. Most abundant numbers are also semiperfect; abundant numbers which are not semiperfect are called weird numbers.

See also

Notes

  1. Caldwell, Chris, "A proof that all even perfect numbers are a power of two times a Mersenne prime".
  2. Dickson, L. E. (1919). History of the Theory of Numbers, Vol. I. Washington: Carnegie Institution of Washington. p. 4.
  3. Commentary on the Gospel of John 28.1.1-4, with further references in the Sources Chrétiennes edition: vol. 385, 58-61.
  4. http://torreys.org/sblpapers2015/S22-05_philonic_arithmological_exegesis.pdf
  5. Roshdi Rashed, The Development of Arabic Mathematics: Between Arithmetic and Algebra (Dordrecht: Kluwer Academic Publishers, 1994), pp. 328–329.
  6. Munich, Bayerische Staatsbibliothek, Clm 14908
  7. Smith, DE (1958). The History of Mathematics: Volume II. New York: Dover. p. 21. ISBN 0-486-20430-8.
  8. Dickson, L. E. (1919). History of the Theory of Numbers, Vol. I. Washington: Carnegie Institution of Washington. p. 10.
  9. Pickover, C (2001). Wonders of Numbers: Adventures in Mathematics, Mind, and Meaning. Oxford: Oxford University Press. p. 360. ISBN 0-19-515799-0.
  10. Peterson, I (2002). Mathematical Treks: From Surreal Numbers to Magic Circles. Washington: Mathematical Association of America. p. 132. ISBN 88-8358-537-2.
  11. All factors of 2p  1 are congruent to 1 mod 2p. For example, 211  1 = 2047 = 23 × 89, and both 23 and 89 yield a remainder of 1 when divided by 11. Furthermore, whenever p is a Sophie Germain prime—that is, 2p + 1 is also prime—and 2p + 1 is congruent to 1 or 7 mod 8, then 2p + 1 will be a factor of 2p  1, which is the case for p = 11, 23, 83, 131, 179, 191, 239, 251, ... A002515.
  12. Song Y. Yan (2009). Primality Testing and Integer Factorization in Public-Key Cryptography. Advances in Information Security 11 (2nd ed.). Springer-Verlag. p. 199. ISBN 0-387-77268-5.
  13. O'Connor, John J.; Robertson, Edmund F., "Abu Ali al-Hasan ibn al-Haytham", MacTutor History of Mathematics archive, University of St Andrews.
  14. "GIMPS Home". Mersenne.org. Retrieved 2013-02-05.
  15. GIMPS Milestones Report. Retrieved 2014-02-24
  16. Weisstein, Eric W., "Perfect Number", MathWorld.
  17. Dickson, L. E. (1919). History of the Theory of Numbers, Vol. I. Washington: Carnegie Institution of Washington. p. 6.
  18. Oddperfect.org.
  19. 1 2 3 Ochem, Pascal; Rao, Michaël (2012). "Odd perfect numbers are greater than 101500" (PDF). Mathematics of Computation 81 (279): 1869–1877. doi:10.1090/S0025-5718-2012-02563-4. ISSN 0025-5718. Zbl pre06051364.
  20. Kühnel, U (1949). "Verschärfung der notwendigen Bedingungen für die Existenz von ungeraden vollkommenen Zahlen". Mathematische Zeitschrift 52: 201–211. doi:10.1515/crll.1941.183.98. Retrieved 30 March 2011.
  21. Roberts, T (2008). "On the Form of an Odd Perfect Number" (PDF). Australian Mathematical Gazette 35 (4): 244.
  22. Grün, O (1952). "Über ungerade vollkommene Zahlen". Mathematische Zeitschrift 55 (3): 353–354. doi:10.1007/BF01181133. Retrieved 30 March 2011.
  23. Nielsen, PP (2003). "An upper bound for odd perfect numbers". Integers 3: A14–A22. Retrieved 30 March 2011.
  24. Goto, T; Ohno, Y (2008). "Odd perfect numbers have a prime factor exceeding 108" (PDF). Mathematics of Computation 77 (263): 1859–1868. doi:10.1090/S0025-5718-08-02050-9. Retrieved 30 March 2011.
  25. Iannucci, DE (1999). "The second largest prime divisor of an odd perfect number exceeds ten thousand" (PDF). Mathematics of Computation 68 (228): 1749–1760. doi:10.1090/S0025-5718-99-01126-6. Retrieved 30 March 2011.
  26. Iannucci, DE (2000). "The third largest prime divisor of an odd perfect number exceeds one hundred" (PDF). Mathematics of Computation 69 (230): 867–879. doi:10.1090/S0025-5718-99-01127-8. Retrieved 30 March 2011.
  27. Nielsen, PP (2015). "Odd perfect numbers, Diophantine equations, and upper bounds" (PDF). Mathematics of Computation 84 (0): 2549–2567. doi:10.1090/S0025-5718-2015-02941-X. Retrieved 13 August 2015.
  28. Nielsen, PP (2007). "Odd perfect numbers have at least nine distinct prime factors" (PDF). Mathematics of Computation 76 (260): 2109–2126. doi:10.1090/S0025-5718-07-01990-4. Retrieved 30 March 2011.
  29. The Collected Mathematical Papers of James Joseph Sylvester p. 590, tr. from "Sur les nombres dits de Hamilton", Compte Rendu de l'Association Française (Toulouse, 1887), pp. 164–168.
  30. http://www.math.harvard.edu/~knill/seminars/perfect/handout.pdf
  31. Makowski, A. (1962). "Remark on perfect numbers". Elem. Math. 17 (5): 109.
  32. Gallardo, Luis H. (2010). "On a remark of Makowski about perfect numbers". Elem. Math. 65: 121–126..
  33. Jones, Chris; Lord, Nick (1999). "Characterising non-trapezoidal numbers". The Mathematical Gazette (The Mathematical Association) 83 (497): 262–263. doi:10.2307/3619053. JSTOR 3619053
  34. Hornfeck, B (1955). "Zur Dichte der Menge der vollkommenen zahlen". Arch. Math. 6 (6): 442–443. doi:10.1007/BF01901120.
  35. Kanold, HJ (1956). "Eine Bemerkung ¨uber die Menge der vollkommenen zahlen". Math. Ann. 131 (4): 390–392. doi:10.1007/BF01350108.
  36. H. Novarese. Note sur les nombres parfaits Texeira J. VIII (1886), 11–16.
  37. Dickson, L. E. (1919). History of the Theory of Numbers, Vol. I. Washington: Carnegie Institution of Washington. p. 25.
  38. Redmond, Don (1996). Number Theory: An Introduction to Pure and Applied Mathematics. Chapman & Hall/CRC Pure and Applied Mathematics 201. CRC Press. Problem 7.4.11, p. 428. ISBN 9780824796969..

References

  • Euclid, Elements, Book IX, Proposition 36. See D.E. Joyce's website for a translation and discussion of this proposition and its proof.
  • Kanold, H.-J. (1941). "Untersuchungen über ungerade vollkommene Zahlen". Journal für die Reine und Angewandte Mathematik 183: 98–109. 
  • Steuerwald, R. "Verschärfung einer notwendigen Bedingung für die Existenz einer ungeraden vollkommenen Zahl". S.-B. Bayer. Akad. Wiss. 1937: 69–72. 

Further reading

External links

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