Niven's Theorem

Niven's Theorem states that if a positive integer is divisible by the sum of its digits, then that integer is a multiple of 9. In simpler terms, when you take any number, add its digits together, and if that sum divides the original number evenly, you can be sure that number is also divisible by 9. This concept helps in quickly checking divisibility by 9 without performing a full division, which is useful in mathematics and number theory.