[dghughes]

Prime numbers are amazing.

I was watching a math documentary and one example was a Cicada in North Carolina that only emerges once every thirteen years millions of them at once. It's a defense mechanism the sheer number overwhelms predators. The Cicada does this also to avoid appearing when another species of Cicada appears to prevent cross breeding.

The other species in the same region emerges every 7 years. The two will only emerge at the same time every 220 years (I think it as).

Smart bugs!

[Retr0spectrum]

I vaguely remember watching the same documentary, so I don't doubt you. But surely they would emerge at the same time every 21 years?

Edit: I looked it up, its 13 and 17 years, giving a 221 year overall cycle. I guess it only really "matters" that the years are coprime.

https://en.wikipedia.org/wiki/Periodical_cicadas

[jweather]

This is a similar concept to the odd numbers of teeth used in gear chains. If you had an 8-tooth and a 16-tooth gear, they will wear unevenly as the same teeth (with potential manufacturing defects) always meet the same teeth. If you change that to 7-tooth and 17-tooth, they will only repeat pairings every 119 teeth and wear will be distributed evenly across all teeth. In general, tooth numbers that are relatively prime (sharing no divisors) are preferred.

[sidhack]

Is this the 3-part BBC documentary - "The Code"?

[dghughes]

Yes I think that's what it is.