Hi, I'm Peter Adamson, and you're listening to the History of Philosophy podcast, brought to with the support of the philosophy department at King's College London and the LMU in Munich,

online at historyof philosophy.net.

Today's episode, Doctors Without Borders, the Republic of Letters.

Here's a story I read recently about some mathematicians who were chatting about numbers over a meal.

One challenged another to recite a long prime number and instantly got the answer, two to the power of 31 minus one. No, said

the first, I want a number where you can recite all the digits. A third mathematician who was

listening said, how about one, two, three, four10-9-87-654-321?

Yes, that extremely memorable number is indeed prime.

The anecdote reminds me of a more famous story about the Indian mathematician Srinivasa Ramuna-Jan.

A colleague who was visiting him had arrived by taxi, and the taxi number

was 1729. When the visitor remarked, apparently struggling to make conversation, that this was rather

a boring number, Ramuna-jan immediately shot back, no, it is a very interesting number. It is the

smallest number expressible as the sum of two cubes in two different ways.

This sort of nerdy fun has been going on since at least the 17th century,

when Maran Mersenne traded mathematical challenges with his friends,

for example, to find two numbers that are each the sum of three squares

and whose sum is also the sum of three squares.

Though there's a significant difference from modern anecdotes,

Moussin and his friends indulged their taste for mathematical puzzles by post.

Moussin wrote a lot of letters.

His surviving correspondence is published in 17 volumes and includes 146 letters he received from Descartes.

In fact, Mersenne is most often thought of as a source of information about his far more famous friend,

...