I'm struggling with taking the gradient with respect to (w.r.t) the matrices $H_R$ and $H_I$ in the following expression

$$\left\| Z - I\odot(H_R^TA + H_I^TB) \right\|_F^2 + \left\|W-\begin{pmatrix} H_R & -H_I\\H_I & H_R\end{pmatrix}P_dS^T \right\|_F^2$$

where $\|\cdot\|_F^2$ represents the Frobenius norm, $\odot$ represents the Hadamard product, $(\cdot)^T$ is the transpose operation, $I$ is the identity matrix, and all the terms in the above expression have the proper dimension.

It would be very helpful to find at least some alternative on how to calculate the gradient w.r.t one of the above-mentioned matrices, say $H_R$, as the same analysis could be then extended to $H_I$. Any help would be highly appreciated.


$\def\R#1{{\mathbb R}^{#1}}\def\v{{\rm vec}}\def\M{{\rm Reshape}}\def\m#1{\left[\begin{array}{r}#1\end{array}\right]}\def\p#1#2{\frac{\partial #1}{\partial #2}}$For ease of typing, replace the subscripted variables with single-letter names $$\eqalign{ R = H_R \qquad Q = H_I \qquad P=P_d \\ }$$ and define the matrices $$\eqalign{ I_2 &= \m{1&0\\0&1}\qquad J_2 = \m{0&-1\\1&0} \\ M &= I\odot(R^TA+Q^TB)- Z \\ N &= (I_2\otimes R+J_2\otimes Q)PS^T - W \\ D &= I\odot M \\ C &= NSP^T \\ }$$ and finally, we'll need the SVD of the last matrix $$\eqalign{ C &= \sum_{k=1}^{rank(C)} \sigma_ku_kv_k^T \\ U_k &= \M(u_k,n,2) \quad&\iff\quad u_k = \v(U_k) \\ V_k &= \M(v_k,n,2) \quad&\iff\quad v_k = \v(V_k) \\ }$$ Let's also use a colon to denote the matrix inner product, i.e. $$\eqalign{ A:B &= {\rm Tr}(A^TB) \\ A:A &= \big\|A\big\|_F^2 \\ }$$ Write the objective function using this new notation. Then calculate its differential. $$\eqalign{ \phi &= M:M + N:N \\ d\phi &= 2M:dM + 2N:dN \\ &= 2M:(I\odot (dR^TA+dQ^TB) + 2N:(I_2\otimes dR+J_2\otimes dQ)PS^T \\ &= 2D:(A^TdR+B^TdQ) + 2C:(I_2\otimes dR+J_2\otimes dQ) \\ &= 2AD:dR + 2C:(I_2\otimes dR) + 2BD:dQ - 2(J_2\otimes I_Q)C:(I_2\otimes dQ) \\ }$$ The terms containing Kronecker products are tricky. Here's where the SVD becomes useful $$\eqalign{ C:(I_2\otimes dR) &= \sum_k\sigma_ku_kv_k^T:(I_2\otimes dR) \\ &= \sum_k\sigma_ku_k:(I_2\otimes dR)v_k \\ &= \sum_k\sigma_ku_k:\v(dR\,V_kI_2) \\ &= \sum_k\sigma_kU_k:dR\,V_k \\ &= \left(\sum_k\sigma_kU_kV_k^T\right):dR \\ &= E:dR \\ }$$ Substitute the SVD result and set $dQ=0$ to obtain the gradient wrt $R$ $$\eqalign{ d\phi &= 2(AD+E):dR \\ \p{\phi}{R} &= 2(AD+E) \\ }$$ A similar calculation for the gradient wrt $Q$ is a bit tricker, but the result is $$\eqalign{ F &= \sum_k\sigma_kU_kJ_2V_k^T \\ \p{\phi}{Q} &= 2(BD-F) \\ }$$

There are other ways (besides the SVD) to handle the Kronecker term, but the formulas are longer/messier.


Here's one way to deal with the Kronecker terms which doesn't require the SVD.

Given matrices of the following dimensions and a cost function $$\eqalign{ &I_p\in\R{p\times p} \qquad X\in\R{m\times n} \qquad Y\in\R{pm\times pn} \\ &\psi = Y:\left(I_p\otimes X\right) \\ }$$ To take advantage of the block-diagonal structure of the RHS, define block-matrix analogs of the standard $\{e_k\}$ basis vectors (i.e. columns of the $I_p$ identity matrix) $$\eqalign{ E_k &= (e_k\otimes I_m) &\in\R{pm\times m} \\ F_k &= (e_k\otimes I_n) &\in\R{pn\times n} \\ }$$ and note that $$\eqalign{ E_j^T(I_p\otimes X)F_k &= \left(e_j^T\otimes I_m\right) \left(I_p\otimes X\right) \left(e_k\otimes I_n\right) \\ &= \left(e_j^Te_k\right)X \\ &= \delta_{jk}X \\ }$$ Evaluate the cost function using block-wise summation and calculate its gradient. $$\eqalign{ \psi &= \sum_{j=1}^p\sum_{k=1}^p\;E_j^TYF_k:E_j^T(I_p\otimes X)F_k \\ &= \sum_{j=1}^p\sum_{k=1}^p\;E_j^TYF_k:\delta_{jk}X \\ &= \sum_{k=1}^p\;E_k^TYF_k:X \\ d\psi &= \sum_{k=1}^p\;E_k^TYF_k:dX \\ \p{\psi}{X} &= \sum_{k=1}^p\;E_k^T Y F_k \\ &= \sum_{k=1}^p\;\left(e_k^T\otimes I_m\right)Y\big(e_k\otimes I_n\big) \\ }$$

  • $\begingroup$ Thank you so much for providing such a thorough answer, also full of valuable insights. Just for the sake of clarifying a particular step in the analysis, when you introduce the SVD representation of the matrix $C$, wouldn't be more convenient to work with the SVD representation of the matrix product $SP^T$ instead? The rationality is based on the fact that $C$ depends upon the expression $N$ which in turn also includes $R$ and $Q$ ($H_R$ and $H_I$ in the original problem), so I wouldn't be able to find any particular way to compute the SVD in this circumstance. Is my analysis correct? $\endgroup$ – Luis Feb 15 at 7:15
  • $\begingroup$ Assume $y=x^2+e^x$. If someone tells you that $\frac{dy}{dx}=2x+(y-x^2)\;$ the appearance of both $y$ and $x$ on the RHS of the formula does not prevent you from using it to calculate the derivative $y$ wrt $x$. Likewise the fact that the SVD depends on $R$, does not prevent you from using it to calculate the gradient wrt $R$. The answer has been updated with an SVD-free method. $\endgroup$ – greg Feb 15 at 11:47
  • $\begingroup$ I got your point, the example you introduced in your comment illustrates this in detail. Thank you one more time for the valuable insights. I know this goes beyond my original question but it would be equally useful to get, if possible, some feedback on it. As you may have noticed from the beginning, the main reason I’m trying to compute the above-mentioned gradients is to ultimately find the values of $R$ and $Q$ that minimize the objective function originally given. (It continues in the next comment) $\endgroup$ – Luis Feb 16 at 3:33
  • $\begingroup$ There are several alternatives one can use to accomplish this, however given the nature of the gradients, i.e., they involve more complex operations such as Hadamard and Kronecker products, I’d be more inclined towards using some sort of iterative approach rather than attempting to directly solve, from the gradients, a system of linear equations in the variables $R$ and $Q$. Do you think this would be the most suitable approach or you’d rather recommend a different one? Thanks in advance. $\endgroup$ – Luis Feb 16 at 10:36
  • $\begingroup$ Since you are simultaneously solving for two variables, I would recommend an ADMM-style iteration, i.e. alternate between a gradient step for R, then a gradient step for Q, then R, then Q, etc $$\eqalign{ R_+ &= R - \alpha\left(\frac{\partial\phi}{\partial R}\right) \\ Q_+ &= Q - \beta\left(\frac{\partial\phi}{\partial Q}\right) \\ }$$ where the scalar step-lengths $(\alpha,\beta)$ minimize the cost function at each iteration. Kronecker and Hadamard products are intrinsic operations in most numerical languages (Julia, NumPy, etc) so don't be too concerned about that aspect. $\endgroup$ – greg Feb 16 at 15:37

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