Is it possible for the directional derivative to exist for every unit vector $\widehat{u}$ at some point $(a, b)$, but $f$ is not differentiable there.
Please help me to solve.
Is it possible for the directional derivative to exist for every unit vector $\widehat{u}$ at some point $(a, b)$, but $f$ is not differentiable there.
Please help me to solve.
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