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In this month's installment of Wrong, But Useful, Colin and Dave are joined by mathematical editor and proofreader @samhartburn.

We apologise for the sound quality. We've done the best we can.

- Sam enjoys @robeastaway's Maths On The Go with her primary-school children.
- Dave plugs Colin's books. It takes us some time to recover from the shock.
**Number of the podcast**: $10^{6400} - 10^{6352} - 1$, a prime number consisting of 6,399 nines and an eight. Colin mentions the idea of Trinity Hall primes, although not by name.
- Sam asked for ideas about what makes a good textbook - Dave has some.
- The 2018 Mathical prizes for mathematical children’s books have been announced:
- Dave has a point about fireworks. Colin gibbers about rainbows.
- Issue 07 of Chalkdust is due out today! Sam has an article in it. Colin and Sam have advance copies, and Dave feels left out.
- Colin has a "$\pi$" day rant. Nobody writes a date as 3.14. We write it as 14/3, which is about 4.67, Feigenbaum's constant.
- Sam's article is about the Menger sponge-cake she baked for Big MathsJam last year.
- Sam also wrote songs for the MathsJam Jam (
*Ever Tried To Divide By Something You Shouldn't've* and *Argand*). Colin wrote *The Signs They Need A-Changin'* about terrible British road signs for football stadiums (although @roice713 points out you can have such shapes in hyperbolic space.)
- Colin is off to G4G next month but needs to crack a code first.
- Dave liked Colin's line: "Complex numbers are all fun and games until someone loses an $i$".
- Dave has incorrectly ranked mathematical writing implements. Colin's correct favourite is a Kuru Toga mechanical pencil in a Leuchtturm1917 notebook. He's also a fan of Staedler fineliner pens.
**Puzzle feedback**: gold stars for @dragon_dodo (Dominika Vasilkova), @chrishazell72 (Chris Hazell) and @alephthought (Chris Hellings) who got the correct answer of 105,263,157,894,736,842.
**Puzzle**: There are eight winning lines in a 3x3 noughts-and-crosses grid, and 49 in a 3x3x3 grid. How many in a 3x3x3x3 grid? Can you generalise?

## Colin

Colin is a Weymouth maths tutor, author of several Maths For Dummies books and A-level maths guides. He started Flying Colours Maths in 2008.
He lives with an espresso pot and nothing to prove.