Here's an interesting problem: an ant traverses a circle with a circumference of one metre at a rate of one centimetre per second. After each second, the circumference of the circle increases by one metre. Will the ant ever return to its starting point?
Let's consider the matter. In the first second, the ant traverses 1% of the circumference; in the second second, it traverses 1/2%; in the third second, 1/3% and so on. The cumulative distance covered is given by the sum of:
1 + 1/2 + 1/3 + 1/4 + 1/5 + ...
This is the harmonic series and it is divergent, meaning that the sum is continuously increasing and never reaches any upper limit. Contrast this to a convergent geometric series, the sum of whose terms approaches ever closer to 2 as more terms are added:
1 + 1/2 + 1/4 + 1/8 + 1/16 + ...
In the case of the harmonic series, the sum will certainly surpass the 100% required for the ant to return to its starting point. It may take a long time but it will happen. In fact OEIS A004080 tells us that it would take the following number of seconds:
15092688622113788323693563264538101449859497
This of course represents more than 4.78 x 10^23 years which is far in excess of the age of the universe! The following screenshot is taken from the WolframAlpha article about the harmonic series.
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| Taken from http://mathworld.wolfram.com/HarmonicSeries.html |
This Numberphile YouTube video was the inspiration for this post.
