13112221

This number is a prime.

The largest known prime in the look-and-say sequence (starting with 1). To generate a member of the sequence from the previous term, "look" and "say" the digits of the previous term, counting the number of digits in groups of the same digit. For example, 111221 has three 1's, two 2's, and then one 1; therefore 111221 is followed by 312211. The sequence begins 1, 11, 21, 1211, 111221, ... . [Coveiro]