# The Life-Changing Magic of Numbers

## Interview with

**Chris Smith interviews Cambridge University mathematician Bobby Seagull about his book, The Life-Changing Magic of Numbers...**

Chris - Bobby, you’ve got a book sitting there, is this it, is this the new tome that you just put out?

Bobby - The life changing magic of numbers.

Chris - When's it out? Just now?

Bobby - So it is just come out this September. Yeah that's right.

Chris - Brilliant. And what's it about?

Bobby - It's partly autobiographical, but it's partly an ode to numbers because I know that as a maths teacher, if I go into a dinner party or meet friends at a pub and say that I love maths, I’m a maths teacher, pretty much the first reaction from most people, not all people and not the esteemed audience here or our listeners, our dear listeners, they'll be like oh God, maths, I hated it, I couldn't do it at school, what's the point of it. And my book is almost like a soft manifesto showing that actually maths is quite cool, quite fun, and appears in unexpected places.

Chris - All right, you've got one minute to convince us that maths is quite cool, quite fun and appears in unexpected places. Take it away.

Bobby - An unexpected place is in dating. I am currently single, and I am trying to use maths to help me to be unsingle.

Chris - That’s not a good opening line is it?

Bobby - No, it’s not! It’s not because of maths, it’s separate, it’s not causation! So firstly, I used an adapted version of Drake’s hypothesis to estimate the number of alien civilizations in our galaxy and …

Chris - If you want to date an alien, you're getting really desperate now Bobby.

Bobby - So again, initially I have used the hypothesis from Drake and according to my own version there are there 73 people I could date in the UK. That doesn't sound good.

Chris - But hang on a minute. So there’s 65 million people in the UK and there are just 75 that suit your parameters. I mean, how stringent are you being?

Bobby - Yeah, I think it's perhaps … the key thing is that I'm someone who likes Verdi, but Stormzy, but Bake Off, but Love Island. So I've got my own issues, but that’s separate. So the interesting part is that I use something called Optimal Stopping Theory to work out how many dates I need to go on.

Chris - And how many is it? Is it within a lifetime? Is this doable?

Bobby - So imagine I say I want to get married in five years, and I'm prepared to go on two dates a week for 50 weeks, that's 100 dates and over five years that’s 500. So actually Optimal Stopping Theory… you know “e” from the exponential? So 1/e gives us roughly 37%. And Optimal Stopping Theory says I need to go on 37% of my total 500 dates. That's a 185 dates, and I can't accept any of them, I have to reject all of them no matter if they love Bake Off and Love Island and University Challenge and then the person after the 185th date that is better than all of the previous ones, she’s the one. Obviously she has to accept as well.

Chris - That sounds like a fallacy though because there's an equal chance that the first date you go on will be your dream lifetime partner, surely, compared with the person who's the one after the 37.5% person.

Bobby - But again we don't know that the person could turn up as the first or the second or the third or...

Chris - But this is what I'm saying, so how do you know.. why are you saying you want to go with the one straight after…

Bobby - This might go better by putting it to pen and paper, but 1/e, 1/2.72 turns out to be, in any sort of decision, say you wanna buy a house and you got a hundred houses to go and look at, or you wanna buy a pair of shoes, it tells you that roughly about a third of the way through you need to discard the first third or 37% and then after, that mathematically you should get the best solution.

Chris - And how far through your hundred eighty five dates are you?

Bobby - About nine.

Chris - You've got a long way to go.

Bobby - I do!

Chris - Okay so you've convinced me that you can overthink things. What's the next fun and fantastic fact that we can learn about in the book?

Bobby - It comes back to cicadas. Because again, my students, we were learning about prime numbers recently and they often say “Oh sir, but Mr. Seagull, prime numbers you only use them in exam, I never see them anywhere in nature”. And then when you brought up, when you mentioned what's the most intelligent insect, and I loved telling my students about the cicadas. And these are little tiny creatures are apparently very tasty. It's only because I've seen them in a documentary and Marcus du Sautoy says they are tasty so I take his word. But again, the fact that they've evolutionarily developed the fact that emerging in 7,13 or 17 year cycles evades predators, I think that's amazing, because, had they picked 12 let's say, then any predator that emerges every 2, 3, 4 or 6 years will eventually overlap with it, and that for me is amazing. It shows actually, mathematics is something that not just humans practice but insects.

Chris - Sounds like a slightly dodgy argument though Bobby because the thing is, nature is very good at finding an opportunity, and maybe you can prop me up on this one Eleanor, but the fact is, if the cicadas have evolved to do that, then surely things that like lunching on cicadas would have evolved convergently to do the same thing wouldn't they? Why haven't they?

Eleanor - It's a really good question and one to which I would love to know the answer, but I don't know of any predators who follow this pattern. I guess in this particular case, you'd have to have a predator who was specialised in eating cicadas. So I'm guessing that they are banking on the fact that you get a lot of generalist predators who wouldn't bother to adapt specifically to eat them. But you never know, they might catch up one day.

Bobby - I think you sort of nailed it on the head there because an entomologist called Stephen Jay Gould said that potential predators often have shorter life cycles of 2 to 5 years so they can't really stretch and keep up with the ones who are greedy enough to save for a 13 or 17 years.

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