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Let $\mu$ and $\nu$ be two probability measures with finite moments (in $\mathcal{P}_2(\mathbb{R})$) equipped with 2-Wasserstein distance. Let $F_\mu$, $F_\nu$ be their cumulative distribution functions respectively. Then it is well known that the optimal transport map from $\mu$ to $\nu$ is given by $T_\mu(x):=F^{-1}_\nu(F_\mu(x))$ provided $\mu$ is absolutely continuous w.r.t. Lebesgue measure. Consider the map $T^\sigma_\mu(x):=F^{-1}_{\nu_\sigma}(F_{\mu_\sigma}(x))$ where $\mu_\sigma:=\mu*N_\sigma$ and $N_\sigma$ is the normal distribution with zero mean and variance $\sigma^2$.

Fixing $\nu$ and $x \in \mathbb{R}$, I am wondering that if $$ \mu \mapsto \int_{\mathbb{R}} T^\sigma_\mu(x+y)\exp(-y^2/2\sigma^2)dy $$ is continuous on $\mu$ in the 2-Wasserstein space. I think that $T^\sigma_\mu(z)$ is continuous on $\mu$ in the 2-Wasserstein space for each $z$. However, I cannot find the corresponding dominator to put the limit insider the integral.

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$\newcommand{\si}{\sigma}\newcommand{\W}{\mathcal W}\newcommand{\R}{\mathbb R} $Yes, the map \begin{equation*} \mu \mapsto J(\mu):=\int_\R T^\si_\mu(x+y)\exp(-y^2/2\si^2)\,dy \tag{10}\label{10} \end{equation*} is continuous on the 2-Wasserstein space, say $\W_2$.

Indeed, let $\mu_n\to\mu$ in $\W_2$ (as $n\to\infty$). Suppose that $X_n\sim\mu_n$ and $X\sim\mu$. Then $X_n\to X$ in distribution and $EX_n^2\to EX^2$. Letting $Z$ be a standard normal random variable independent of the $X_n$'s and $X$, we get $X_n+\si Z\to X+\si Z$ in distribution, so that \begin{equation*} F_{(\mu_n)_\si}\to F_{\mu_\si} \tag{20}\label{20} \end{equation*} pointwise. Also, the function $F_{\nu_\si}$ is continuous and strictly increasing, so that the inverse function $F^{-1}_{\nu_\si}$ is continuous (and strictly increasing). So, by \eqref{20}, \begin{equation*} T_{\mu_n}^\si\to T_\mu^\si \tag{30}\label{30} \end{equation*} pointwise.

By Markov's inequality, $F_{\nu_\si}(z)\le C/z^2$ for real $z<0$, where $C:=\int_\R x^2\,\nu(dx)<\infty$, and hence \begin{equation*} |F^{-1}_{\nu_\si}(u)|\le\sqrt{C/u} \tag{40}\label{40} \end{equation*} if $0<u<F_{\nu_\si}(0)$ (and $F_{\nu_\si}(0)\in(0,1)$). On the other hand, letting $m_n$ and $m$ denote the medians of $(\mu_n)_\si$ and $\mu_\si$, respectively, and letting $\Phi_\si$ denote the cdf of $N(0,\si^2)$, we see that \eqref{20} implies $m_n\to m$ and hence \begin{equation*} F_{(\mu_n)_\si}(z)=\int_\R F_{\mu_n}(z-y)d\Phi_\si(y) \\ \ge\int_{-\infty}^{z-m_n}F_{\mu_n}(m_n)d\Phi_\si(y) =\frac12\,\Phi_\si(z-m_n)=\exp\left(-\frac{z^2}{2\si^2+o(1)}\right) \end{equation*} uniformly in $n$ as $z\to-\infty$. So, by \eqref{40}, \begin{equation*} |T_{\mu_n}^\si(z)|=|F^{-1}_{\nu_\si}(F_{(\mu_n)_\si}(z))| \le \exp\frac{z^2}{4\si^2+o(1)} \tag{50}\label{50} \end{equation*} uniformly in $n$ as $z\to-\infty$. Similarly, \eqref{50} holds in the right-tail zone, that is, uniformly in $n$ as $z\to\infty$. Also, in view of \eqref{20} and because $F_{\mu_\si}$ is strictly increasing, we see that \begin{equation*} |T_{\mu_n}^\si(z)|=O(1) \end{equation*} uniformly in $n$ for $|z|=O(1)$. So, in view of \eqref{50}, \begin{equation*} |T_{\mu_n}^\si(z)|=O\Big(\exp\frac{z^2}{3\si^2}\Big) \end{equation*} uniformly in $n$ and real $z$. So, for each fixed real $x$, \begin{equation*} |T^\si_{\mu_n}(x+y)|\exp(-y^2/2\si^2) =O\Big(\exp-\frac{y^2}{7\si^2}\Big) \end{equation*} uniformly in $n$ and real $y$.

Thus, by \eqref{10}, \eqref{30}, and dominated convergence, $J(\mu_n)\to J(\mu)$. $\quad\Box$

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  • $\begingroup$ Thank you so much for the clear answer! Do you think it is also jointly continuous in $x$ and $\mu$? $\endgroup$
    – mnmn1993
    Commented Jan 26 at 15:19
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    $\begingroup$ @mnmn1993 : Yes, it is also jointly continuous in $x$ and $\mu$: replace $X_n$ and $X$ by $X_n-x$ and $X-x$, and note that the domination holds uniformly over all $x$ in any bounded set. $\endgroup$ Commented Jan 26 at 15:27

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