15 (fifteen) is the natural number following 14 and preceding 16. 15 as the difference of two positive squares (in orange). 15 is: The eighth composite number and the sixth semiprime and the first odd and fourth discrete semiprime; [ 1 ] its proper divisors are 1, 3, and 5, so the first of the form (3.q), [ 2 ] where q is a higher prime. More @Wikipedia
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