The derivative of $v^Tv$ with respect to v is $$\frac{\partial (v^Tv)}{\partial v}=2v.$$Suppose v is a unit vector, then $v^Tv=1$ and $$2v=\frac{\partial (v^Tv)}{\partial v}=\frac{\partial (1)}{\partial v}=0.$$However, $v$ is not a zero vector. So what is wrong of my derivation? Thanks a lot.
2026-04-04 14:17:32.1775312252
What is the derivative of the norm of a unit vector?
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