421

This number is a prime.

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

Puzzio: 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 the smallest titanic "Hailstone prime?" The sequence begins 421, 63105168421, 1263105168421, etc. [Honaker, 1999]

Submitted: 1999-09-17 21:52:39;   Last Modified: 2020-06-20 20:05:27.
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