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Definition/Summary
A perfect number is a number which is the sum of its proper divisors (half the sum of its total divisors). Even perfect numbers are a Mersenne prime times a power of two; odd perfect numbers are not known to exist.
Equations
Sum-of-divisors function:
\sigma(n)=\sum_{k|n}k
\sigma(p^aq^b)=\sigma(p^a)\sigma(q^b)\;\;(p,q\text{ relatively prime})
\sigma(p^a)=\frac{p^{a+1}-1}{p-1}
Definition of N perfect:
2N=\sigma(N)
Form of an even perfect number:
N=M_p(M_p+1)/2=2^{p-1}(2^p-1)
where M_p is a Mersenne prime.
Extended explanation
The first two perfect numbers are:
6 = 1 + 2 + 3 = 2^{2-1} (2^2-1)
28 = 1 + 2 + 4 + 7 + 14 = 2^{3-1} (2^3-1)
The next two are:
496 = 2^{5-1} (2^5-1)
8128 = 2^{7-1} (2^7-1)
* This entry is from our old Library feature. If you know who wrote it, please let us know so we can attribute a writer. Thanks!
A perfect number is a number which is the sum of its proper divisors (half the sum of its total divisors). Even perfect numbers are a Mersenne prime times a power of two; odd perfect numbers are not known to exist.
Equations
Sum-of-divisors function:
\sigma(n)=\sum_{k|n}k
\sigma(p^aq^b)=\sigma(p^a)\sigma(q^b)\;\;(p,q\text{ relatively prime})
\sigma(p^a)=\frac{p^{a+1}-1}{p-1}
Definition of N perfect:
2N=\sigma(N)
Form of an even perfect number:
N=M_p(M_p+1)/2=2^{p-1}(2^p-1)
where M_p is a Mersenne prime.
Extended explanation
The first two perfect numbers are:
6 = 1 + 2 + 3 = 2^{2-1} (2^2-1)
28 = 1 + 2 + 4 + 7 + 14 = 2^{3-1} (2^3-1)
The next two are:
496 = 2^{5-1} (2^5-1)
8128 = 2^{7-1} (2^7-1)
* This entry is from our old Library feature. If you know who wrote it, please let us know so we can attribute a writer. Thanks!