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A (normalized) gradient vector $\hat{g}$ at a point $x$ defines a hyperplane. Given a vector $v$ the operator below reflects it against the hyperplane $$ R(v, \hat{g}) = v - 2\hat{g}\hat{g}^\top v $$

enter image description here

Now suppose I have a Jacobian matrix $J_f$ instead of a gradient vector. What is the generalization of this operation?

My Attempt

Let $f:\mathbb{R}^n\to\mathbb{R}^m$ be a function such that $f(x) = (f_1(x), \ldots, f_m(x))^\top$. Its Jacobian is a matrix $$ J_f(x) = \begin{pmatrix} \nabla f_1(x)^\top \\ \vdots \\ \nabla f_m(x)^\top \end{pmatrix} $$ Now each of its rows could be used to do a reflection with respect to that gradient $R(v, \widehat{\nabla f}_i(x))$, as shown in the picture above. However, I would like to generalize this operation to the whole Jacobian.

Idea 1

One way I thought of doing this is by using the reflection with respect to each of the coordinates sequentially. That is I keep on iterating $$ v \longleftarrow R(v, \widehat{\nabla f}_i(x)) $$ for $i=1, \ldots, m$. However the order in which you do this operation seems to matter.

Idea 2

Perhaps rather than reflecting against each gradient, one could sum up all the gradients, normalize and reflect against a "mean" gradient? That is one would compute $$ \overline{\nabla f}(x) = \sum_{i=1}^m \nabla f_i(x) $$ then normalize it and compute $R(v, \widehat{\overline{\nabla f}})$

Euler_Salter
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  • The Jacobian being a matrix, its kernel defines a linear subspace. Since the Jacobian of a numerical function can be identified with its gradient by $df(v) = \langle \nabla f, v\rangle$, these two objects reflects the same thing – Didier Jan 17 '22 at 16:03
  • @Didier Thank you for the comment. I am a bit confused (mainly because I am misunderstanding something). Suppose $h:\mathbb{R}^n\to\mathbb{R}$. Then the gradient $g(x) = \nabla h(x)$ is in $\mathbb{R}^n$ and $R(v, \hat{g})$ describes the reflection of $v$ with respect to the tangent space of $h$ at $x$. $$

    $$ Now for $f:\mathbb{R}^n\to\mathbb{R}^m$ the Jacobian is a matrix $J_f\in\mathbb{R}^{m\times n}$. I am not sure what mathematical object this matrix defines. What do you mean by $\nabla h(x)^\top v$?

    – Euler_Salter Jan 17 '22 at 16:08
  • I said that they coincide for numerical function, that is function with values in $\Bbb R$. Of course, the gradient is not defined for functions with values in greater dimensional spaces – Didier Jan 17 '22 at 16:17
  • @Didier I added some explanation in my post! – Euler_Salter Jan 17 '22 at 16:33
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    https://math.stackexchange.com/questions/156880/what-does-it-mean-to-take-the-gradient-of-a-vector-field/4359170#4359170 – tryst with freedom Jan 17 '22 at 16:55
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    The general notion is called Householder transformation, where there is no "gradient vector", not even a map $f$. What kind of generalization are you looking for? –  Jan 17 '22 at 16:56

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