# Rate of convergence of probabilities under Wasserstein convergence of measures?

Suppose $\mu=\sum_{i=1}^k p_i\delta_{x_i}$ and $\mu'=\sum_{i=1}^k p_i'\delta_{x_i'}$ are two atomic probability measures. Let $W_r(\mu,\mu')$ be the Wasserstein distance between $\mu$ and $\mu'$. Are there any known bounds of the form $$\sup_i\inf_j|p_i-p_j'| \le C\cdot W_r(\mu,\mu')$$

for some constant $C$ that may depend on quantities such as the number of atoms $k$? It is not hard to prove such a bound for the largest difference between the atoms $\sup_i\inf_jd(x_i,x_j')$, however, I am not sure if it is possible to derive something similar for the probabilities.

Another more concrete perspective: If $W_r(\mu_n,\mu')\to0$ and $\mu_n,\mu'$ all have exactly $k$ atoms, then it is a standard result that there are permutations $\pi_n$ such that $x_{\pi_n(i),n}\to x_i'$ and $p_{\pi(i),n}\to p_i'$. Now suppose $W_r(\mu_n,\mu')\le r_n$ for some sequence $r_n>0$. Does it follow that the rate of convergence of the probabilities is also $r_n$? This is easy to show for the atoms, but I am not sure about the probabilities.

• I'm not quite sure I understand the question. Say I take $x_1 = 1, x_2 = 17$, and $x_1' = 42, x_2' = 1+\epsilon$, with $p_1 = 1, p_2 = 0, p_1'=0, p_2'=1$? Then $\mu = \delta_1$ and $\mu' = \delta_{1+\epsilon}$, so that the Wasserstein distance tends to 0, but $|p_1-p_1'| = |p_2-p_2'|=1$ so the LHS of your proposed inequality seems to just be 1. – Nate Eldredge May 17 '18 at 0:53
• A thing to keep in mind is that Wasserstein distance is a metric for weak convergence, and not for total variation convergence. So you can't omit all consideration of the distances between the atoms. – Nate Eldredge May 17 '18 at 0:55
• @NateEldredge Sorry, this should all be up to some permutation (or alternatively, use the Hausdorff metric on the two sets of probabilities). I clarified this above. In your example, the LHS is now zero. Also, it is okay if the upper bound depends on the distance between the atoms. – JohnA May 17 '18 at 1:09
• How about $\mu = \delta_0$, $\mu' = \frac{1}{2} \delta_{-\epsilon} + \frac{1}{2} \delta_{\epsilon}$? The LHS is $\frac{1}{2}$ and again the Wasserstein distance is arbitrarily small. – Nate Eldredge May 17 '18 at 1:44
• So $\mu = \epsilon \delta_{17} + (1-\epsilon) \delta_0$ and $\mu'$ as before. – Nate Eldredge May 17 '18 at 2:12