homework and exercises - Passing from curl to vector product

I don't understand how to obtain second equation with first part in the equation $$\nabla \times \vec A_0 e^{-j \vec k\cdot \vec r} = -j\vec k\times \vec A_0 e^{-j \vec k\cdot \vec r}.$$ Can you show me how to derive it?

Answer

$$\begin{equation} \boldsymbol{\nabla}\equiv \begin{bmatrix} \dfrac{\partial\hphantom{ x_{1}}}{\partial x_{1}}\vphantom{\dfrac{\dfrac{}{}}{}}\\ \dfrac{\partial\hphantom{ x_{1}}}{\partial x_{2}}\vphantom{\dfrac{\dfrac{}{}}{}}\\ \dfrac{\partial\hphantom{ x_{1}}}{\partial x_{3}}\vphantom{\dfrac{\dfrac{}{}}{}} \end{bmatrix}\,, \quad \mathbf{A}= \begin{bmatrix} A_{1}\vphantom{\dfrac{\dfrac{}{}}{}}\\ A_{2}\vphantom{\dfrac{\dfrac{}{}}{}}\\ A_{3}\vphantom{\dfrac{\dfrac{}{}}{}}\\ \end{bmatrix}\,, \quad \mathbf{k}\boldsymbol{\cdot}\mathbf{x}=k_{1}x_{1}+k_{2}x_{2}+k_{3}x_{3} \tag{01} \end{equation}$$

$$\begin{align} \boldsymbol{\nabla}\boldsymbol{\times}\left(\mathbf{A}\,e^{\boldsymbol{-}\mathrm{i}\:\mathbf{k}\boldsymbol{\cdot}\mathbf{x}}\right) & = \begin{bmatrix} \mathbf{e}_{1} & \mathbf{e}_{2} & \mathbf{e}_{3}\vphantom{\dfrac{\dfrac{}{}}{}}\\ \dfrac{\partial\hphantom{ x_{1}}}{\partial x_{1}} & \dfrac{\partial\hphantom{ x_{1}}}{\partial x_{2}} & \dfrac{\partial\hphantom{ x_{1}}}{\partial x_{3}}\vphantom{\dfrac{\dfrac{}{}}{}}\\ A_{1}e^{\boldsymbol{-}\mathrm{i}\:\mathbf{k}\boldsymbol{\cdot}\mathbf{x}} & A_{2}e^{\boldsymbol{-}\mathrm{i}\:\mathbf{k}\boldsymbol{\cdot}\mathbf{x}} & A_{3}e^{\boldsymbol{-}\mathrm{i}\:\mathbf{k}\boldsymbol{\cdot}\mathbf{x}} \vphantom{\dfrac{\dfrac{}{}}{}} \end{bmatrix} = \begin{bmatrix} A_{3}\dfrac{\partial e^{\boldsymbol{-}\mathrm{i}\:\mathbf{k}\boldsymbol{\cdot}\mathbf{x}}}{\partial x_{2}}-A_{2}\dfrac{\partial e^{\boldsymbol{-}\mathrm{i}\:\mathbf{k}\boldsymbol{\cdot}\mathbf{x}}}{\partial x_{3}}\vphantom{\dfrac{\dfrac{}{}}{}}\\ A_{1}\dfrac{\partial e^{\boldsymbol{-}\mathrm{i}\:\mathbf{k}\boldsymbol{\cdot}\mathbf{x}}}{\partial x_{3}}-A_{3}\dfrac{\partial e^{\boldsymbol{-}\mathrm{i}\:\mathbf{k}\boldsymbol{\cdot}\mathbf{x}}}{\partial x_{1}}\vphantom{\dfrac{\dfrac{}{}}{}}\\ A_{2}\dfrac{\partial e^{\boldsymbol{-}\mathrm{i}\:\mathbf{k}\boldsymbol{\cdot}\mathbf{x}}}{\partial x_{1}}-A_{1}\dfrac{\partial e^{\boldsymbol{-}\mathrm{i}\:\mathbf{k}\boldsymbol{\cdot}\mathbf{x}}}{\partial x_{2}}\vphantom{\dfrac{\dfrac{}{}}{}} \end{bmatrix} \nonumber\\ & =\boldsymbol{-}\mathrm{i}\:e^{\boldsymbol{-}\mathrm{i}\:\mathbf{k}\boldsymbol{\cdot}\mathbf{x}} \begin{bmatrix} k_{2}A_{3}-k_{3}A_{2}\vphantom{\dfrac{\dfrac{}{}}{}}\\ k_{3}A_{1}-k_{1}A_{3}\vphantom{\dfrac{\dfrac{}{}}{}}\\ k_{1}A_{2}-k_{2}A_{1}\vphantom{\dfrac{\dfrac{}{}}{}} \end{bmatrix} =\boldsymbol{-}\mathrm{i}\:e^{\boldsymbol{-}\mathrm{i}\:\mathbf{k}\boldsymbol{\cdot}\mathbf{x}} \underbrace{ \begin{bmatrix} \mathbf{e}_{1} & \mathbf{e}_{2} & \mathbf{e}_{3}\vphantom{\dfrac{\dfrac{}{}}{}}\\ k_{1} & k_{2} & k_{3}\vphantom{\dfrac{\dfrac{}{}}{}}\\ A_{1} & A_{2} & A_{3}\vphantom{\dfrac{\dfrac{}{}}{}}\vphantom{\dfrac{\dfrac{}{}}{}} \end{bmatrix}}_{\mathbf{k}\boldsymbol{\times}\mathbf{A}} \tag{02} \end{align}$$ so $$\begin{equation} \boxed{\:\boldsymbol{\nabla}\boldsymbol{\times}\left(\mathbf{A}\,e^{\boldsymbol{-}\mathrm{i}\:\mathbf{k}\boldsymbol{\cdot}\mathbf{x}}\right) =\boldsymbol{-}\mathrm{i}\:e^{\boldsymbol{-}\mathrm{i}\:\mathbf{k}\boldsymbol{\cdot}\mathbf{x}}\left(\mathbf{k}\boldsymbol{\times}\mathbf{A}\right)\:\vphantom{\dfrac12^{\tfrac12}_{\tfrac12}}} \tag{03} \end{equation}$$

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More generally :

If $\:\psi\left(x_{1},x_{2},x_{3}\right)\:$ and $\:\mathbf{A}\left(x_{1},x_{2},x_{3}\right)\:$ are scalar and vector functions respectively of the coordinates in $\:\mathbb{R}^{3}\:$ then $$\begin{equation} \boxed{\:\boldsymbol{\nabla}\boldsymbol{\times}\left(\psi\mathbf{A}\right)=\boldsymbol{\nabla}\psi\boldsymbol{\times}\mathbf{A}+\psi\boldsymbol{\nabla}\boldsymbol{\times}\mathbf{A}\:\vphantom{\dfrac12^{\tfrac12}_{\tfrac12}}} \tag{04} \end{equation}$$ Equation (03) is a special case of (04) with $$\begin{equation} \psi\left(\mathbf{x}\right)\equiv e^{\boldsymbol{-}\mathrm{i}\:\mathbf{k}\boldsymbol{\cdot}\mathbf{x}}\,,\quad \mathbf{A}\left(\mathbf{x}\right)=\text{constant} \tag{05} \end{equation}$$ and the fact that $$\begin{equation} \boldsymbol{\nabla}\psi=\boldsymbol{\nabla} e^{\boldsymbol{-}\mathrm{i}\:\mathbf{k}\boldsymbol{\cdot}\mathbf{x}}=\boldsymbol{-}\mathrm{i}\:e^{\boldsymbol{-}\mathrm{i}\:\mathbf{k}\boldsymbol{\cdot}\mathbf{x}}\mathbf{k} \tag{06} \end{equation}$$

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^{Proof of identity (04): $$\begin{align} \boldsymbol{\nabla}\boldsymbol{\times}\left(\psi\mathbf{A}\right) & = \begin{bmatrix} \mathbf{e}_{1} & \mathbf{e}_{2} & \mathbf{e}_{3}\vphantom{\dfrac{\dfrac{}{}}{}}\\ \dfrac{\partial\hphantom{ x_{1}}}{\partial x_{1}} & \dfrac{\partial\hphantom{ x_{1}}}{\partial x_{2}} & \dfrac{\partial\hphantom{ x_{1}}}{\partial x_{3}}\vphantom{\dfrac{\dfrac{}{}}{}}\\ \psi A_{1} & \psi A_{2} & \psi A_{3} \vphantom{\dfrac{\dfrac{}{}}{}} \end{bmatrix} = \begin{bmatrix} \dfrac{\partial \left(\psi A_{3}\right)}{\partial x_{2}}-\dfrac{\partial \left(\psi A_{2}\right)}{\partial x_{3}}\vphantom{\dfrac{\dfrac{}{}}{}}\\ \dfrac{\partial \left(\psi A_{1}\right)}{\partial x_{3}}-\dfrac{\partial \left(\psi A_{3}\right)}{\partial x_{1}}\vphantom{\dfrac{\dfrac{}{}}{}}\\ \dfrac{\partial \left(\psi A_{2}\right)}{\partial x_{1}}-\dfrac{\partial \left(\psi A_{1}\right)}{\partial x_{2}}\vphantom{\dfrac{\dfrac{}{}}{}} \end{bmatrix} = \begin{bmatrix} A_{3}\dfrac{\partial \psi}{\partial x_{2}}+ \psi\dfrac{\partial A_{3}}{\partial x_{2}}-A_{2}\dfrac{\partial \psi}{\partial x_{3}}-\psi\dfrac{\partial A_{2}}{\partial x_{3}}\vphantom{\dfrac{\dfrac{}{}}{}}\\ A_{1}\dfrac{\partial \psi}{\partial x_{3}}+ \psi\dfrac{\partial A_{1}}{\partial x_{3}}-A_{3}\dfrac{\partial \psi}{\partial x_{1}}-\psi\dfrac{\partial A_{3}}{\partial x_{1}}\vphantom{\dfrac{\dfrac{}{}}{}}\\ A_{2}\dfrac{\partial \psi}{\partial x_{1}}+ \psi\dfrac{\partial A_{2}}{\partial x_{1}}-A_{1}\dfrac{\partial \psi}{\partial x_{2}}-\psi\dfrac{\partial A_{1}}{\partial x_{2}}\vphantom{\dfrac{\dfrac{}{}}{}} \end{bmatrix} \nonumber\\ & = \begin{bmatrix} A_{3}\dfrac{\partial \psi}{\partial x_{2}}-A_{2}\dfrac{\partial \psi}{\partial x_{3}}\vphantom{\dfrac{\dfrac{}{}}{}}\\ A_{1}\dfrac{\partial \psi}{\partial x_{3}}-A_{3}\dfrac{\partial \psi}{\partial x_{1}}\vphantom{\dfrac{\dfrac{}{}}{}}\\ A_{2}\dfrac{\partial \psi}{\partial x_{1}}-A_{1}\dfrac{\partial \psi}{\partial x_{2}}\vphantom{\dfrac{\dfrac{}{}}{}} \end{bmatrix} + \begin{bmatrix} \psi\dfrac{\partial A_{3}}{\partial x_{2}}-\psi\dfrac{\partial A_{2}}{\partial x_{3}}\vphantom{\dfrac{\dfrac{}{}}{}}\\ \psi\dfrac{\partial A_{1}}{\partial x_{3}}-\psi\dfrac{\partial A_{3}}{\partial x_{1}}\vphantom{\dfrac{\dfrac{}{}}{}}\\ \psi\dfrac{\partial A_{2}}{\partial x_{1}}-\psi\dfrac{\partial A_{1}}{\partial x_{2}}\vphantom{\dfrac{\dfrac{}{}}{}} \end{bmatrix} = \underbrace{ \begin{bmatrix} \mathbf{e}_{1} & \mathbf{e}_{2} & \mathbf{e}_{3}\vphantom{\dfrac{\dfrac{}{}}{}}\\ \dfrac{\partial \psi}{\partial x_{1}} & \dfrac{\partial\psi}{\partial x_{2}} & \dfrac{\partial\psi}{\partial x_{3}}\vphantom{\dfrac{\dfrac{}{}}{}}\\ A_{1} & A_{2} & A_{3} \vphantom{\dfrac{\dfrac{}{}}{}} \end{bmatrix}}_{\boldsymbol{\nabla}\psi\boldsymbol{\times}\mathbf{A}} + \psi\, \underbrace{ \begin{bmatrix} \mathbf{e}_{1} & \mathbf{e}_{2} & \mathbf{e}_{3}\vphantom{\dfrac{\dfrac{}{}}{}}\\ \dfrac{\partial\hphantom{ x_{1}}}{\partial x_{1}} & \dfrac{\partial\hphantom{ x_{1}}}{\partial x_{2}} & \dfrac{\partial\hphantom{ x_{1}}}{\partial x_{3}}\vphantom{\dfrac{\dfrac{}{}}{}}\\ A_{1} & A_{2} & A_{3} \vphantom{\dfrac{\dfrac{}{}}{}} \end{bmatrix}}_{\boldsymbol{\nabla}\boldsymbol{\times}\mathbf{A}} \nonumber\\ & =\boldsymbol{\nabla}\psi\boldsymbol{\times}\mathbf{A}+\psi\boldsymbol{\nabla}\boldsymbol{\times}\mathbf{A} \tag{04} \end{align}$$}