312

This number is a composite.

312 is the only number m less than 30000 such that prime(k*m) + k*m is prime for k = 1, 11, 1111, 111111, 11111111, and 111111111. [Firoozbakht]

312 = prime(3*1*2) * phi(3*1*2) * sigma(3*1*2). It is the only known number with this property (up to 10^8). [Firoozbakht]

312 is the average of a twin prime pair. Describe 312 to find another, i.e., 312 is 'one 3, one 1, one 2' or 131112. This is the smallest example. [Honaker]