M = 15 The 15 perfect matchings of K6 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. a deficient number, a lucky number, a bell number (i.e., the number of partitions for a ...