# Spectral norm of a Hadamard product

Let $F$ be the $n\times n$ DFT matrix, i.e., $F_{i,j} = \exp\left(-\tfrac{2\pi(i-1)(j-1)}{n}\imath\right)$, for $1\le i,j\le n$ and $\imath =\sqrt{-1}$. Futhermore, let $\Vert \cdot\Vert$ and $\odot$ denote the spectral norm and the Hadamard product, respectively.

For any $X\in \mathbb{R}^{n\times n}$, is there an explicit formula for $$\Vert F\odot X\Vert, \tag{\star}$$ in terms of $\Vert X\Vert$ or other simple functions of $X$? If instead of the spectral norm we had the Frobenius norm then the answer simply would have been the Frobenius norm of $X$. For $(\star)$, howerver, I guess the answer is negative. If my guess is correct, is there a reasonably sharp upper bound for $(\star)$?

I obtained the following upper bound. Let $D_z$ denote a diagonal matrix whose diagonal entries are given by vector $z$. Furthermore, write the SVD of $X$ as $X=UD_\sigma V^\mathrm{T}=\sum _{i=1}^r\sigma_i u_iv_i^\mathrm{T}$ where $r$ denotes the rank of $X$. Then we have \begin{align*} F\odot X &=\sum_{i=1}^r F\odot \left(\sigma_i u_i v_i^\mathrm{T}\right)\\ &= \left[\begin{array}{cccc} D_{u_1}&D_{u_2}&\dotsm&D_{u_r} \end{array}\right] \left(D_\sigma \otimes F\right)\left[\begin{array}{c} D_{v_1}\\ D_{v_2}\\ \vdots\\ D_{v_r} \end{array}\right]\\ &=\left[\begin{array}{cccc} \sigma_1D_{u_1}&\sigma_2D_{u_2}&\dotsm&\sigma_rD_{u_r} \end{array}\right] \left(I_{r\times r}\otimes F\right)\left[\begin{array}{c} D_{v_1}\\ D_{v_2}\\ \vdots\\ D_{v_r} \end{array}\right], \end{align*} where $\otimes$ denotes the Kronecker product. Now we can bound $\left\Vert F\odot X\right \Vert$ as \begin{align*} \left\Vert F\odot X\right\Vert & \leq \left\Vert\left[\begin{array}{cccc} \sigma_1D_{u_1}&\sigma_2D_{u_2}&\dotsm&\sigma_rD_{u_r} \end{array}\right]\right\Vert\left\Vert \left(I_{r\times r}\otimes F\right)\left[\begin{array}{c} D_{v_1}\\ D_{v_2}\\ \vdots\\ D_{v_r} \end{array}\right]\right\Vert\\ &=\sqrt{n} \left\Vert UD_\sigma \right\Vert_{\infty,2}\left\Vert V\right\Vert_{\infty,2}, \end{align*} where $\left\Vert\cdot\right\Vert_{\infty,2}$ is the maximum row-wise $\ell_2$-norm. There is also a simpler bound \begin{align*} \left\Vert F\odot X\right\Vert&\leq \left\Vert F\odot X\right\Vert_F\\ &=\left\vert X\right\Vert_F\\ &=\left\Vert\sigma\right\Vert_2, \end{align*} which can be sharper.