Difference between revisions of "Perfect number"
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==Even Perfect Numbers== | ==Even Perfect Numbers== | ||
| − | <math>n</math> is an even perfect number if and only if <math>n=\frac{p(p+1)}{2}</math>, where <math>p</math> is a [[prime number]] of the form <math>2^k-1</math> for some <math>k</math>. | + | <math>n</math> is an [[even integer | even]] perfect number if and only if <math>n=\frac{p(p+1)}{2}</math>, where <math>p</math> is a [[prime number]] of the form <math>2^k-1</math> for some <math>k</math>. |
Primes of the form <math>2^k-1</math> are called [[Mersenne prime]]s. | Primes of the form <math>2^k-1</math> are called [[Mersenne prime]]s. | ||
===Proof=== | ===Proof=== | ||
| − | + | It is conjectured that there are are infinitely many Mersenne primes and hence infinitely many even perfect numbers. No [[odd integer | odd]] perfect numbers are known, and any that do exist must be greater than <math>10^{500}</math>. It is conjectured that there are none. No one has been able to prove or disprove these conjectures. | |
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| − | It is conjectured that there are are infinitely many Mersenne primes and hence infinitely many even perfect numbers. No odd perfect numbers are known, and any that do exist must be greater than <math>10^{500}</math>. It is conjectured that there are none. No one has been able to prove or disprove these conjectures. | ||
Revision as of 16:43, 12 October 2006
A positive integer 
is called a perfect number if it is the sum of its proper divisors.
The first four perfect numbers are 
, 
, 
, and 
. These were the only perfect numbers known to ancient mathematicians.
Even Perfect Numbers
is an even perfect number if and only if 
, where 
is a prime number of the form 
for some 
.
Primes of the form are called Mersenne primes.
Proof
It is conjectured that there are are infinitely many Mersenne primes and hence infinitely many even perfect numbers. No odd perfect numbers are known, and any that do exist must be greater than 
. It is conjectured that there are none. No one has been able to prove or disprove these conjectures.
