Thursday, May 14, 2009

15

15 = 3 x 5.

15 is a triangular number, the sum of the first five positive integers: 1 + 2 + 3 + 4 + 5 = 15.

7 can be partitioned in
15 ways.

A three-by-three
magic square has the magic constant 15 (the sum of each row, column, or diagonal).

For
n greater than 15, at least one number between n and 2n is the product of three different primes.

15 has a unique representation as a sum of four squares: 15 = 12 + 12 + 22 + 32.

15 is the smallest number that has three representations as a sum of three primes: 15 = 2 + 2 + 11= 3 + 5 + 7 = 5 + 5 + 5.


15 is the atomic number of the element phosphorus.

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