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Perfect number
In mathematics, a perfect number is defined as an integer which is the sum of its proper positive divisors, excluding itself.
Six (6) is the first perfect number, because 1, 2 and 3 are its proper positive divisors and 1 + 2 + 3 = 6. The next perfect number is 28 = 1 + 2 + 4 + 7 + 14. The next perfect numbers are 496 and 8128.
These first four perfect numbers were the only ones known to the ancient Greeks.
Even perfect numbers
Euclid discovered that the first four perfect numbers are generated by the formula 2^{n1}(2^{n}1):
 for n = 2: 2^{1}(2^{2}  1) = 6
 for n = 3: 2^{2}(2^{3}  1) = 28
 for n = 5: 2^{4}(2^{5}  1) = 496
 for n = 7: 2^{6}(2^{7}  1) = 8128
Noticing that 2^{n}1 is a prime number in each instance, Euclid proved that the formula 2^{n1}(2^{n}1) gives an even perfect number whenever 2^{n}1 is prime.
Ancient mathematicians made many assumptions about perfect numbers based on the four they knew. Most of the assumptions were wrong. One of these assumptions was that since 2, 3, 5, and 7 are precisely the first four primes, the fifth perfect number would be obtained when n = 11, the fifth prime. However, 2^{11}1 = 2047 = 23 * 89 is not prime and therefore n = 11 does not yield a perfect number. Two other wrong assumptions were:
 The fifth perfect number would have five digits since the first four had 1, 2, 3, and 4 digits respectively.
 The perfect numbers would alternately end in 6 or 8.
The fifth perfect number ([math]\displaystyle{ 33\,550\,336=2^{12}(2^{13}1) }[/math]) has 8 digits, thus debunking the first assumption. For the second assumption, the fifth perfect number indeed ends with a 6. However, the sixth (8 589 869 056) also ends in a 6. It has been shown that the last digit of any even perfect number must be 6 or 8.
In order for [math]\displaystyle{ 2^n1 }[/math] to be prime, it is necessary that [math]\displaystyle{ n }[/math] should be prime. Prime numbers of the form 2^{n}1 are known as Mersenne primes, after the seventeenthcentury monk Marin Mersenne, who studied number theory and perfect numbers.
Two millennia after Euclid, Leonhard Euler proved that the formula 2^{n1}(2^{n}1) will yield all the even perfect numbers. This proof was never published during his lifetime but after his death it has been published in 1849. Thus, every Mersenne prime will yield a distinct even perfect number  there is a concrete onetoone association between even perfect numbers and Mersenne primes. This result is often referred to as the "EuclidEuler Theorem."
Only finitely many Mersenne primes are presently known, and it is unknown whether there are infinitely many of them. Thus it also remains uncertain whether there are infinitely many even perfect numbers.
Every even perfect number is a triangular number. Since any even perfect number has the form 2^{n−1}(2^{n}1), it is the sum of all natural numbers up to 2^{n}1. This follows from the general formula stating that the sum of the first m positive integers equals (m^{2} + m)/2. Furthermore, any even perfect number except the first one is the sum of the first 2^{(n1)/2} odd cubes:
 [math]\displaystyle{ 6 = 2^1(2^21) = 1+2+3 }[/math]
 [math]\displaystyle{ 28 = 2^2(2^31) = 1+2+3+4+5+6+7 = 1^3+3^3 }[/math]
 [math]\displaystyle{ 496 = 2^4(2^51) = 1+2+3+\cdots+29+30+31 = 1^3+3^3+5^3+7^3 }[/math]
 [math]\displaystyle{ 8128 = 2^6(2^71) = 1+2+3+\cdots+125+126+127 = 1^3+3^3+5^3+7^3+9^3+11^3+13^3+15^3 }[/math]
Another interesting fact is that the reciprocals of the factors of a perfect number add up to 2:
 For 6, we have [math]\displaystyle{ 1/6 + 1/3 + 1/2+ 1/1 = 2 }[/math]
 For 28, we have [math]\displaystyle{ 1/28 + 1/14 + 1/7 + 1/4 + 1/2 + 1/1 = 2 }[/math]
 etc.
Odd perfect numbers
It is unknown whether there are any odd perfect numbers. Various results have been obtained, but none that have helped to locate one or otherwise resolve the question of their existence.
Any odd perfect number N must satisfy the following conditions:
 N is of the form
 [math]\displaystyle{ N=q^{\alpha} p_1^{2e_1} \ldots p_k^{2e_k}, }[/math]
 where q, p_{1}, …, p_{k} are distinct primes and q ≡ α ≡ 1 (mod 4) (Euler).
 N is bigger than 10^{300} (Richard P. Brent et al. 1991).
 N has at least 8 distinct prime factors (and at least 11 if it is not divisible by 3) (Peter Hagis).
 N has at least 75 prime factors in total, including repetitions (Kevin Hare, 2005).
 N has at least one prime factor greater than 10^{7}, two prime factors greater than 10^{4}, and three prime factors greater than 100.
 N is less than [math]\displaystyle{ 2^{4^{n}} }[/math] where n is the number of distinct prime factors.
 N is of the form 12j + 1 or 36j + 9 (Jacques Touchard).
See also
References
 Kevin Hare, New techniques for bounds on the total number of prime factors of an odd perfect number. Preprint, 2005. Available from his webpage.
 R. P. Brent, G. L. Cohen and H. J. J. te Riele, Improved techniques for lower bounds for odd perfect numbers. Mathematics of Computation 57 (1991), 857868. Available from Brent's webpage.
External links
 David Moews: Perfect, amicable and sociable numbers
 Perfect numbers  History and Theory
 Perfect Number  from MathWorld
 List of Perfect Numbers: A000396 at the OnLine Encyclopedia of Integer Sequences
 Perfect number
General numbers 
Special numbers 
Prime numbers 
