Exercise 9.26

Letting $f(x,y)=g(x)$ be the function given in the example, then $D_{2}f(x,y)=0$, so $D_{12}f(x,y)=0$, for all $(x,y)$. However, $D_{1}f(x,y)=g^{\prime }(x)$ does not exist. For $g$ you can use the function defined in Theorem 7.18.