According to Pythagoras, numerical perfection depended on a number’s divisors (numbers that will divide perfectly into the original one). For instance, the divisors of 12 are 1, 2, 3, 4, and 6. When the sum of a number’s divisors is greater than the number itself, it is called an “excessive” number. Therefore 12 is an excessive number because its divisors add up to 16. On the other hand, when the sum of a number’s divisors is less than the number itself, it is called “defective.” So 10 is a defective number because its divisors (1, 2, and 5) add up to only 8. The most significant and rarest
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