Number:15 (Meta, no ontology)

From Compile Worlds

15 is the first magic number which is not also a prime number. However, this is because 15 = 2⁴-1, and the exponent 4 is not prime. The first non-prime magic number whose exponent is itself prime is in fact 2¹¹-1 = 2,047 = 23 × 89.

15 is also the third tetramagic number, and the last which is small enough to care about, as the fourth is 65,535 and the fifth is way too freaking big to write here.