Prime Curios!: 421

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.

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]

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