42

This number is a composite.

The 13th case of a prime concatenation of the form n & n-th prime is with n = 42 and n-th prime = 181 (42181). Coincidentally, 181 is congruent to 13 modulo 42. Note that 18142181 is also prime. Furthermore the operations 181 - 42 and 42 + 181 yield primes resp. 139 and 223 leading us to these prime combinations with 42 in the middle : 13942223 (A) and 22342139 (B). Finally (B)(A) = 2234213913942223 is prime as well. [De Geest]