3599

This number is a composite.

Numbers consisting of 35 concatenated with an even number of 9's have factors (6*10^(n/2)-1 and (6*10^(n/2)+1). Thus 359(w) (w any numbers of 9's) can be prime only if w is odd.

The emirpimes 3599 is the product of twin primes, one of which is its middle two digits: 3599 = 59 * 61; 9953 = 37 * 269. [Post]