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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?