39

This number is a composite.

The least composite odd number that is the sum of the primes between its smallest and largest prime factors (39 = 3 * 13 = 3 + 5 + 7 + 11 + 13). [Rivera]

Concatenating the 39th row numbers of Pascal's triangle forms a prime. [Rivera]

39 is the smallest positive integer which cannot be formed from the first four primes (used once each), using only the simple operations +, -, *, / and ^. [Hartley]

39 = 3*9 + π(39) [Firoozbakht]

π(39) = 3 + 9. [Gupta]

39 = nextprime(3+9)*π(π(3+9)). The two numbers 39 and reversal(nextprime(39)) are the only numbers with this property. [Hasler]