421

This number is a prime.

All integers ever tested in the Collatz problem eventually ends in the sequence 4-2-1.

The starting number 6 generates a Collatz trajectory of 6-3-10-5-16-8-4-2-1 and via concatenation (63105168421) we get a prime. Can you find a titanic "hailstone prime?" The sequence begins 421, 63105168421, 1263105168421, etc. [Honaker]