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]

39 is the product of the first two primes ending with '3' and the sum of the first three primes ending with '3' (39 = 3 * 13 = 3 + 13 + 23) [Sladcik]

The sum of first 39 Sloof Lirpa primes is another Sloof Lirpa prime. Smallest case. [Sariyar]

The smallest Sloof Lirpa prime. A Sloof Lirpa (April Fools spelled backwards) prime is an April Fools prime that results in a different April Fools prime when its decimal digits are reversed. The sequence begins 39, 93, 117, 123, 129, 143, 147, 153, 159, 169, 177, 183, 187, 189, 309, 319, 321, … .