- #1
- 19,941
- 11,006
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:
[tex]\sigma(n)=\sum_{k|n}k[/tex]
[tex]\sigma(p^aq^b)=\sigma(p^a)\sigma(q^b)\;\;(p,q\text{ relatively prime})[/tex]
[tex]\sigma(p^a)=\frac{p^{a+1}-1}{p-1}[/tex]
Definition of N perfect:
[tex]2N=\sigma(N)[/tex]
Form of an even perfect number:
[tex]N=M_p(M_p+1)/2=2^{p-1}(2^p-1)[/tex]
where M_p is a Mersenne prime.
Extended explanation
The first two perfect numbers are:
6 = 1 + 2 + 3 = [itex]2^{2-1} (2^2-1)[/itex]
28 = 1 + 2 + 4 + 7 + 14 = [itex]2^{3-1} (2^3-1)[/itex]
The next two are:
496 = [itex]2^{5-1} (2^5-1)[/itex]
8128 = [itex]2^{7-1} (2^7-1)[/itex]
* 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:
[tex]\sigma(n)=\sum_{k|n}k[/tex]
[tex]\sigma(p^aq^b)=\sigma(p^a)\sigma(q^b)\;\;(p,q\text{ relatively prime})[/tex]
[tex]\sigma(p^a)=\frac{p^{a+1}-1}{p-1}[/tex]
Definition of N perfect:
[tex]2N=\sigma(N)[/tex]
Form of an even perfect number:
[tex]N=M_p(M_p+1)/2=2^{p-1}(2^p-1)[/tex]
where M_p is a Mersenne prime.
Extended explanation
The first two perfect numbers are:
6 = 1 + 2 + 3 = [itex]2^{2-1} (2^2-1)[/itex]
28 = 1 + 2 + 4 + 7 + 14 = [itex]2^{3-1} (2^3-1)[/itex]
The next two are:
496 = [itex]2^{5-1} (2^5-1)[/itex]
8128 = [itex]2^{7-1} (2^7-1)[/itex]
* 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!