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All integers ever tested in the Collatz problem eventually ends in the sequence 421. Collatz Challenge: The starting number 6 generates a Collatz trajectory of (63105168421) and via concatenation (63105168421) we get a prime. Can you find the first 421 "Hailstone primes?" The sequence begins 421, 63105168421, 1263105168421, etc. [Honaker, 1999] Jeu du 421 (quatrecentvingtetun) is a French dice game. The first prime formed by the powers of 2 in numerical order from right to left. [Luhn] The sum of the letters of "prime number" if we use the alphaprime code. [Zirkle] A vigesimal repunit: 400 + 20 + 1 = 20^2 + 20^1 + 20^0 = 111_20. [McCandless] The only nontitanic prime concatenated in reverse order from consecutive powers of 2. [Loungrides] The product of first 421 positive even numbers minus 1, i.e., 2^421*421!1 is the smallest titanic prime of form 2^p*p!1, where p is a prime. [Loungrides] The concatenation of the number of 1digit primes and the number of 2digit primes. [Gaydos]
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