f is not differentiable at all irrational numbers.
All sequences of irrational numbers (ai≠ x0)_（i=1〜∞ ） converging to the irrational point x0 imply a constant sequence (f(ai)=0)_（i=1〜∞ ）, identical to 0,
and so lim _（i→ ∞ ）｜ (f(ai)-f(x0)）／（ai-x0））｜=0.
According to Hurwitz's theorem, there also exists a sequence of rational numbers (bi=ki／i)_（i=1〜∞ ）,
converging to x0, with (ki∈ Z ,i∈ N )） (ki∈ Z ,i∈ N )） coprime and |ki／i-x0|＜ 1／（√ 5）* i^2）.
Thus for all i,: ｜（f(bi)-f(x0)）／ （bi-x0）｜ ＞ （1／i - 0）／（1／(（√ 5）* i^2)））= √ 5* i ≠ 0 and so f is not differentiable at all irrational x0. (引用終り)
