Edinburgh is a city with a pretty good bus network. And if you mention this to almost anyone who lives here, they are likely to go off on a rant about how incredibly stupid it is that they are currently digging up half the streets in the city to put down tram lines. Apparently, according to several taxi drivers who have talked at me about the trams, it's one of the best bus networks in the world. Or maybe it was Europe. It doesn't really matter to our topic today, which is not going to be the trams, but rather prime numbers.

From time to time, I take the bus down to Leith, an area of Edinburgh were several of our friends live. As I get to my bus stop, I will usually have a look to see which routes that stop at that particular stop, and then I will check the route map in the bus shelter to see which of those buses go to Leith. And as it happens, the buses that go to Leith from my bus stop is precisely those whose route numbers are divisible by 7. For a while, I thought that perhaps this was a general rule, that all buses divisible by 7 go to Leith, but this turned out not to be the case. For example, the 22 goes to Leith, whereas the 42 doesn't.

Still, this got me thinking, and I had soon devised a clever system for numbering buses. First, you divide your city into areas, and then you assign each area a prime number. And then, in order to figure out the route number of a bus, just multiply together the numbers of each area it goes through. If we had used this system, and Leith had the number 7, then you would be able to tell at a glance that the 79534 goes through Leith, wereas the 34086 doesn't. Pretty clever, if I might say so myself.

-Tor Nordam