A "perfect number" is one in which the sum of the divisors of the number add up to the number. The first "perfect number" is 6, because its divisors are 1,2,3. What is the second perfect number?

Following the rule that makes 6 the first perfect number, we can test the integers that follow it. The number 28 fits the definition perfectly. Its proper divisors, which are all the numbers that can divide it evenly besides itself, are 1, 2, 4, 7, and 14. When you add these divisors together, their sum is 1 + 2 + 4 + 7 + 14, which equals 28. Because the sum of its parts equals the whole, 28 earns its title as the second perfect number.

These special numbers have captivated mathematicians for thousands of years, dating back to the ancient Greeks. The mathematician Euclid discovered a formula for generating them around 300 B.C. They are exceptionally rare; after 6 and 28, the next perfect number is 496, and the one after that is 8,128. They grow in size very quickly, and to date, only 51 have ever been found, each one corresponding to a special type of prime number.

Interestingly, every perfect number discovered so far has been an even number. Whether an odd perfect number can exist is one of the oldest and most famous unsolved problems in all of mathematics. It shows that even in a concept as simple as adding up divisors, deep and fascinating mysteries can still be hiding.