Author Robin Sloan offered a simple competition:

give me a prime number of your choosing. I’ll send books to the five people who choose the lowest unique prime numbers. So, if you pick 2 but seven other people pick 2, no book for you. If you pick 3 and no one else picks 3, you get a book.

Which number would you pick?

The primes people chose, including the five winners, and more background information are given in a blog post The Penumbra primes.

Since the primes are countable, this is a lowest unique bid auction with slightly different psychology.

Probably go for 83.

Pick 87 and hope the person running the auction doesn’t realize.

@Paul: I thought almost the same thing when I saw the link! A set of rules is only as good as its weakest link, and you’ll almost certainly be low and unique (but high enough that it’s not obvious whether it’s a prime).

I say 89, because the number of primes between 1 and 89^2 is exactly 1000, as amplified upon here: