By the time you read this, the BMC at Queen Mary will be over, or almost over.

Due to circumstances more-or-less outside my control, I was only able to attend the first half-day. I heard the plenary lecture by Cédric Villani, the Google lecture, and the public lecture on Martin Gardner by Persi Diaconis. But I also had plenty of opportunity to talk to old friends, and to hear the news that this meeting reversed the trend of recent years and was the biggest BMC ever, with over 300 delegates. My colleagues, especially Ivan Tomašić, had done an absolutely marvellous job of getting the show on the road, with a stunning list of invited speakers. There was a real buzz on the first day, and I hope it continued throughout the meeting.

I will say a little about the one event in which I had some part, the public lecture. When the BMC business meeting in 2011 accepted the invitation to meet at Queen Mary this year, I looked for something to celebrate, and found the ideal subject: this year is Martin Gardner’s 100th anniversary. He died four years ago, but his memory is certainly alive.

Martin Gardner was a mathematical magician. I mean this in two senses. First, there is a connection between mathematics and magic; many magic tricks are based on a piece of mathematics, and some areas of mathematics lend themselves readily to the creation of mathematical tricks. But second (and for me, far more important), Gardner could take the straw of everyday objects or events, and spin solid gold mathematics out of it, as he showed many times in his famous *Scientific American* column.

So my next thought is that the person to speak about this should also be a mathematical magician in both those senses. The obvious person who sprang to mind was Persi Diaconis. As an added bonus, Persi had known Martin Gardner very well over a long period. The moment he agreed to come and give a public lecture was when I was sure that the meeting would be a success.

The lecture was held in the recently refurbished Great Hall of the People’s Palace in the East End of London, now part of QMUL. The hall had a capacity of 750, and the event was “sold out” (tickets were actually free), so many Martin Gardner fans had come expecting a treat. And what a beautiful lecture it was. Persi set out to show us the kind of person Martin was, and did this mostly by telling stories. The audience, many of whom had come under the spell of Martin Gardner, were privileged to be able to feel they knew him much better at the end.

For me, the most poignant story was one Persi told in answer to the last question. Apparently, Martin Gardner, on being asked by a publisher (the son of his publisher at the time, in fact) whether he had any books which could be considered, answered that he had in his desk drawer a novel he had written some time earlier. It was duly brought out, and Persi described how Martin had been eagerly anticipating its reception by the critics, and even speculating that this could be the break that would get him off the treadmill of writing about mathematical diversions. Stop and think for a moment what a disaster that would have been! In the end, the book was largely ignored by the critics and the book-buying public, and Gardner continued doing what he did so well. Persi’s comment was, “I’m a mathematician, and if I ever try to be anything else, I hope someone throws a pie in my face”.

Persi did one of Martin Gardner’s best-known magic tricks for us. He got Ursula Martin to be his assistant, and invited her to answer three questions about a “random” card, where she was allowed to lie to any or all of the questions; he still correctly. Mathematically, it shouldn’t work, and I didn’t speak to anyone afterwards who figured out how it was done (or if they did, they were keeping quiet).

Anyway, I am allowed to use a much-misused word and say that the audience had been treated to a unique experience; Persi announced afterwards that he would almost certainly never give that lecture again. How fortunate we were to be there!

I worked out how the trick was done. But I’m not telling you!

I really enjoyed the account of the lecture! But I didn’t understand the description of the trick specifically. From Peter’s text I get the following: the volunteer chooses a random card. Persi asks three questions, but in each case the volunteer is allowed to answer anything she likes, whether true or false. On the basis of the answers, Persi names the card. That certainly sounds impossible (except for sleight-of-hand things like forcing a particular choice of card, or spying on the card chosen, in which case the questions are just a distraction). Is there more to it than that?

Yes, the volunteer confirmed her answer by counting out cards, one for each letter in the answer.

I really wanted to come, at least to the group theory workshop, but it did not work out. Is there any chance to find videos of the lectures online?

Some of the lectures were videoed, I am not sure which ones or how long they will take getting them up. Cedric’s was, Persi’s was not.

The “volunteer” writes: it was a wonderful lecture, especially the slide where PD went from a simple shuffling problem to the Artin Conjecture, which you have explained in your next post. As to the trick, as the assistant standing on the stage, and with little expertise in handling playing cards, I was the last person to figure out how it was done, as I was for too busy trying not to drop the cards and thus looking an idiot in front of 750 people! Which is no doubt part of the magician’s skill. PD said in the lecture that it was a version of a trick devised by Martin Gardner, called the “Lie speller”. A trick on similar principles is described here. http://celebrationofmind.org/wp-content/uploads/2013/09/seven-card-trick-4.pdf

I don’t think PD can really have meant that about never repeating that lecture – you can catch it again in Portland in August: http://www.maa.org/meetings/mathfest/program-details/2014/invited-addresses