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, 327, 329, 339, 341, 351, 357, 369, 381, 387, 399, 711, 723, 729, 741, 753, 759, 771, 781, 783, 789, 903, 913, … .