Revisiting The Ant 


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Revisiting The Ant  2012/02/20So last time in The Ant And The Rubber Band we were talking about an infinitely patient ant walking on an infinitely stretchy rubber band. If you haven't already, you'll need to read that. So here's what's happening:
If you continue to work methodically like this, and don't make any mistakes (so it's best to write a spreadsheet or a program or something) then you can work out where the ant is at any moment. The problem is, working methodically with the numbers doesn't always provide the insight, especially if you program a computer to do it. Sometimes working the numbers for yourself, by hand, allows the insight to come. And the insight is in the percentages. For the first minute the ant walks 1 cm, which is 1% of the length. In the second minute it walks 1 cm, which is 1/2 % of the length. Then it walks 1 cm which is 1/3 % of the length. And so on. So the distance travelled, as a percentage, after n minutes is given by the series:
But you may need a lot of terms. Next time I'll explore how we know this is unbounded, what it looks like, and some surprising results that follow from it.
So in fact this works to show that if you have any list of reals then you can construct a real that's not on the list. If we start with, say, the natural numbers, or the reciprocals of the primes, or rationals whose denominators are powers of 5, or the numbers that arise as solutions of polynomials, then the same technique can show that there is a real missing in every interval.
CommentsI've decided no longer to include comments directly via the Disqus (or any other) system. Instead, I'd be more than delighted to get emails from people who wish to make comments or engage in discussion. Comments will then be integrated into the page as and when they are appropriate. If the number of emails/comments gets too large to handle then I might return to a semiautomated system. We'll see.

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