68

This number is a composite.

68 is the smallest composite number that becomes prime when turned upside down. Has the world been turned upside down? [Honaker]

The largest known number that is the sum of two distinct primes in exactly two ways: 68 = 7 + 61 = 31 + 37. [Rivera]

68 = -53+19+43+59. If -53 is considered to be a prime, then the sum of any three of these 4 primes is the square of a prime number; specifically the squares of the first 4 odd primes. [Rupinski]

68 = π(7)*prime(7). Note that 7 is the mean average of the digits of 68. [Worrom]