I see the always-intriguing Collatz conjecture going around a bit again on **Twitter**(as it seems to every few months), but just started wondering what the history/background of it is, which I’ve never seen much about, other than that it originated with Lothar Collatz maybe in the 1930s(?).

The simple statement of it, is that you take any positive integer and apply the following 2 rules iteratively:

- If the number is even, divide it by two, or
- If the number is odd, triple it and add one. (Then repeat.)

Doing so successively you will

*always*conclude with a sequence of integers ending at 4, 2, 1 (...or so goes the conjecture).
People write a lot about the conjecture and continue to work on it,

**what I’m wondering now is how did Collatz stumble upon***but***those**two specific iterative rules to begin with out of essentially an infinite number that might be imagined (even if many would pretty obviously not lead to anything interesting)? Or, you could even come up with__3__iterative rules! Or, or, or… Did he try LOTS of others… have other people since tried LOTS of others? Is there something unique about his two rules, as opposed to ANY others that might be concocted and have some interesting result?
Anyone know, or can point to some informative links?

...And for anyone who's missed it, here's a nice

...And for anyone who's missed it, here's a nice

**introduction to the Collatz conjecture:***Numberphile*

__ADDENDUM:__
In the comments below Brian Hayes responds with

**this link**to an old piece he wrote for**Scientific American**on the subject. Like other pieces, it’s largely analysis of the conjecture, written in Brian’s always-superb exposition, but there is a bit of history on page 12. He also references a piece by Lothar himself, but what I found most interesting in tracking it down, was seeing a number of folks say that though Lothar explored many iterative functions, he never actually claimed specific credit for the so-called 3N+1 problem that took on his own name!
And with all that said, what I’m still not clear about is whether the two conjecture rules involved in 3N+1 were arrived at primarily by sheer trial-and-error, or was there a more methodological/quantitative approach to hitting upon them?

## 1 comment:

A little history midway through this old piece of mine: http://bit-player.org/bph-publications/SciAm-1984-01-Hayes-hailstone.pdf.

Best source: The Ultimate Challenge: The 3x+1 Problem, ed. by Jeff Lagarias, published by AMS, reprints an article by Collatz himself "On the Motivation and Origin of the (3n+1)-Problem."

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