# Why is Minkowski's question mark function denoted by a question mark?

There are some real odd names for functions in mathematics, but Minkowski's question mark function, denoted by $?(x)$, may be the oddest one I have ever seen.

In Zur Geometrie der Zahlen, Minkowski doesn't discuss his choice of notation (at least, not in the relevant sections that I could find and use a translator on).

Why is the function denoted by $?(x)$? Most related graphs of it (I've played around with some) fail to show anything like a question mark.

• Previously the factorial function was denoted by ! . Maybe Minkowski was inspired by that. – Gerald Edgar Aug 17 '15 at 1:52