Solution
= Solution
Put $w=t\phi$ into the differentiability expansion. The remainder is $o(|t|\|\phi\|)$, so division by $t$ and passage to zero gives \b[$g'(0)=DI(u)\phi$]. For $\phi=0$ this is immediate as well.
= Solution
Put $w=t\phi$ into the differentiability expansion. The remainder is $o(|t|\|\phi\|)$, so division by $t$ and passage to zero gives \b[$g'(0)=DI(u)\phi$]. For $\phi=0$ this is immediate as well.