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（以前のスレから関連抜粋）
スレ46 https://rio2016.5ch.net/test/read.cgi/math/1510442940/398
＜引用＞
http://www.unirioja.es/cu/jvarona/downloads/Differentiability-DA-Roth.pdf
DIFFERENTIABILITY OF A PATHOLOGICAL FUNCTION, DIOPHANTINE APPROXIMATION, AND A REFORMULATION OF THE THUE-SIEGEL-ROTH THEOREM JUAN LUIS VARONA 2009
This paper has been published in Gazette of the Australian Mathematical Society, Volume 36, Number 5, November 2009, pp. 353{361.
(抜粋)
So, in this paper we
are going to analyze the dierentiability of the real function
fν(x) =0 if x ∈ R ＼ Q,
　　or =1/q^ν if x = p/q ∈ Q, irreducible,
for various values of ν ∈ R.

Theorem 1. For ν > 2, the function fν is discontinuous (and consequently
not dierentiable) at the rationals, and continuous at the irrationals. With
respect the dierentiability, we have:
(a) For every irrational number x with bounded elements in its continued fraction expansion, fν is differentiable at x.
(b) There exist infinitely many irrational numbers x such that fν is not differentiable at x.
Moreover, the sets of numbers that fulfill (a) and (b) are both of them uncountable.

Theorem 2. For ν > 2, let us denote
Cν = { x ∈ R : fν is continuous at x },
Dν = { x ∈ R : fν is dierentiable at x }.
Then, the Lebesgue measure of the sets R ＼ Cν and R ＼ Dν is 0, but the four sets Cν, R ＼ Cν, Dν, and R ＼ Dν are dense in R.
(引用終り)

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