The book is now available! 421
(another Prime Pages' Curiosity)
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+ All integers ever tested in the Collatz problem eventually ends in the sequence 4-2-1.

Update: Starting with 6, or 12, will produce primes as well. E.g., 6 gives a Collatz trajectory of {6,3,10,5,16,8,4,2,1}, and 63105168421 is prime, etc. Can you find the next starting number that will produce a prime by concatenating the entire trajectory?

+ Jeu du 421 (quatre-cent-vingt-et-un) 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 non-titanic 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]

(There are 4 curios for this number that have not yet been approved by an editor.)

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