I thought I might be continuing my apparent effort to convert all triads into fourfolds, but I’ll take a little break to bring you this mathematical fourfold for the Collatz Conjecture. The Collatz algorithm starts with any specific nonzero integer, then iterates in the following way to turn the number into the next number to operate on, and so on. If X is even, then replace X with X/2. If X is odd, then replace X with 3*X + 1. If you get to 4, then you get 2, then 1, but you next go back to 4. In fact, if you have any power of 2, you quickly drop to 4 and so cycle. The Collatz Conjecture is that if you start with any number, then you will eventually reach 4. (Actually the conjecture is that you will reach 1, but since I’m partial to 4, I’ll state it in this way since it’s pretty much the same thing.)

The conjecture is not proven, but has been shown to be true for every number up to some very large numbers. Some numbers can jump around from larger to smaller to larger again, or up and down and back up, for quite a few steps before being reduced to the 4-2-1-4 sequence. That’s why these sequences are also called hailstone numbers.

One might think of the Collatz algorithm as a model of a toy universe in the following way. The procedure starts with a given number, which for the toy universe would be its initial state. As time moves from instant to instant, the procedure operates to halve the number, or multiply by three and add one. So instant after instant the procedure operates, making the number smaller, or larger as required. The conjecture, if true, would mean that no matter how it starts, our toy universe would always wind down and cycle endlessly through the sequence of 4-2-1-4-2-1-4-2-1-4…

Further Reading:

http://en.wikipedia.org/wiki/Collatz_conjecture

http://xkcd.com/710/

https://www.popularmechanics.com/science/math/a29033918/math-riddle-collatz-conjecture/

https://www.quantamagazine.org/mathematician-terence-tao-and-the-collatz-conjecture-20191211/

[*8.88]

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