When Douglas Adams was asked whether he invented this question because six times nine is actually 54, which is 42 when written in base thirteen, he replied: “I may be a sorry case, but I don't write jokes in base 13.”
This denial by Douglas Adams is well known among his fans, see eg. “How did Douglas Adams choose the Ultimate Question?” on Sci Fi SE.
On the other hand, a friend told me that Stanisław Lem, on the other hand, did write a joke that is based on a positional number system with an unusual radix (not necessarily 13), somewhere in the stories about Captain Pirx.
Is this true? Or, even if not all the details match, is there something close to this in Lem's writings that spawned this rumour? Where do I find the relevant part in the writings?
I've read all the stories about Captain Pirx, but I've read most of them only once or twice, and have probably missed this detail. I admit they're not my most favorite part of Lem's sci fi.