# Academic reference concerning Minkowski's question mark function

The Wikipedia article about Minkowski's question mark function says that "the derivative vanishes on the rational numbers", i.e. that $?(x)=0$ for all $x\in\mathbb Q$. However, I have not been able to find an academic paper or book which contains this result. (I have only found it stated that the derivative is zero almost everywhere, but not that the "almost everywhere" includes all the rationals.) Since I need to refer to this fact in an academic paper of my own, Wikipedia won't do. Can anyone help me with a source?

• please do not use ? for function, it easier if you use $f(x)$ or $g(x)$ like any normal person thanks – terrace Feb 3 '17 at 1:18
• @terrace: $?(x)$ is the standard name of that specific function. – Rahul Feb 3 '17 at 1:19
• well then they are trolling – terrace Feb 3 '17 at 1:22
• @terrace ?(x) is minkowski's famous question mark function. Unless Minkowski was the first recorded troll, born before trolling was even a thing; I think it's safe to rule out the conclusion they are "trolling". – user335907 Feb 3 '17 at 21:58
• @Casper Also, I'd say Minkowski's seminal work introducing the ? function is probably your best bet. – user335907 Feb 3 '17 at 22:00

(ADDED NEXT DAY) Regarding your request for a specific reference that the Minkowski $?(x)$ function has a zero derivative at each rational number, the (apparently unpublished) manuscript On the Minkowski measure by Linas Vepštas (arXiv:0810.1265, October 2008) states the following in its Abstract: One can show by classical techniques that its derivative must vanish on all rationals. As for a specific reference to a published proof, I suggest writing to some of the authors who have published in this area, such as Alkauskas, Bibiloni, Dushistova, Moshchevitin, Paradís, Viader, etc.