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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 2n-1(2n-1):

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

Noticing that 2n-1 is a prime number in each instance, Euclid proved that the formula 2n-1(2n-1) gives an even perfect number whenever 2n-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, 211-1 = 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 ($\displaystyle{ 33\,550\,336=2^{12}(2^{13}-1) }$) 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 $\displaystyle{ 2^n-1 }$ to be prime, it is necessary that $\displaystyle{ n }$ should be prime. Prime numbers of the form 2n-1 are known as Mersenne primes, after the seventeenth-century monk Marin Mersenne, who studied number theory and perfect numbers.

Two millennia after Euclid, Leonhard Euler proved that the formula 2n-1(2n-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 one-to-one association between even perfect numbers and Mersenne primes. This result is often referred to as the "Euclid-Euler 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 2n−1(2n-1), it is the sum of all natural numbers up to 2n-1. This follows from the general formula stating that the sum of the first m positive integers equals (m2 + m)/2. Furthermore, any even perfect number except the first one is the sum of the first 2(n-1)/2 odd cubes:

$\displaystyle{ 6 = 2^1(2^2-1) = 1+2+3 }$
$\displaystyle{ 28 = 2^2(2^3-1) = 1+2+3+4+5+6+7 = 1^3+3^3 }$
$\displaystyle{ 496 = 2^4(2^5-1) = 1+2+3+\cdots+29+30+31 = 1^3+3^3+5^3+7^3 }$
$\displaystyle{ 8128 = 2^6(2^7-1) = 1+2+3+\cdots+125+126+127 = 1^3+3^3+5^3+7^3+9^3+11^3+13^3+15^3 }$

Another interesting fact is that the reciprocals of the factors of a perfect number add up to 2:

• For 6, we have $\displaystyle{ 1/6 + 1/3 + 1/2+ 1/1 = 2 }$
• For 28, we have $\displaystyle{ 1/28 + 1/14 + 1/7 + 1/4 + 1/2 + 1/1 = 2 }$
• 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
$\displaystyle{ N=q^{\alpha} p_1^{2e_1} \ldots p_k^{2e_k}, }$
where q, p1, …, pk are distinct primes and q ≡ α ≡ 1 (mod 4) (Euler).
• N is bigger than 10300 (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 107, two prime factors greater than 104, and three prime factors greater than 100.
• N is less than $\displaystyle{ 2^{4^{n}} }$ where n is the number of distinct prime factors.
• N is of the form 12j + 1 or 36j + 9 (Jacques Touchard).