11115...11513 (29-digits)

This number is a prime.

                                             111151111 5111315117 1151111513

With a prime number of digits, and a prime sum of digits (59), after the trivial "11" this is the smallest prime concatenation of initial terms of the product of distinct primes of form 4k+1 that divide n. The next such prime is 1111511115111315117115111151311295111175137113541111, and next term after that has more than 1000 digits. [Post and Hasler]