Mathematics
Perfect Numbers and Their Mysterious History
Quick fact
Every known perfect number is even and ends in 6 or 28. No one has ever found an odd perfect number, and it's a famous unsolved problem whether any exist.
Why this is interesting
Picture a number that is the sum of its own parts. The number 6 is perfect: 1+2+3=6. But such numbers are so rare that only 51 are known—so what makes them so special?