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]

(There are 4 curios for this number that have not yet been approved by an editor.)

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