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We consider the following probability measure on $\mathbb{R}^2$: $\mu = Leb\vert_{[0,1]} \times \delta_0$. Furthermore the following dilation, say $d$, is defined as $(x,0) \mapsto \frac{1}{2}(\delta_{(x,f(x))}+\delta_{(x,-f(x))})$ where $f$ is a non-continuous function function. Define the probability measure $\nu$ as $\nu = \mu d$. Why is this kernel/dilation not a Lipschitz kernel?

Short remark about Lipschitz kernel: A kernel $d:x\mapsto \theta_x$ (disintegration) transporting $\mu$ to $\mu d$ is called Lipschitz if there exists a subset of $\mathbb{R}^d$ of full $\mu$-measure such that $d$ restricted to this subset is Lipschitz of constant $1$ from $(\mathbb{R}^d,\vert\vert \cdot\vert\vert_{\mathbb{R}^d}$) to $(\mathcal{P}(\mathbb{R}^d),W)$, where $\mathcal{P}(\mathbb{R}^d)$ are the probability measures on $\mathbb{R}^d$ and $W$ is the Kantorovich distance between $\theta$ and $\theta'$ defined by

$W(\theta, \theta') = \sup_{f\text{ } 1-\text{Lipschitz}} \vert\vert \int f d\theta - \int f d\theta'\vert\vert_{\mathbb{R}^d}$.

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  • $\begingroup$ Sorry, but I do not see why this would help me... $\endgroup$
    – tubmaster
    Commented May 14, 2017 at 7:41
  • $\begingroup$ When you write "this kernel/dilation", I assume you mean the mapping $x\mapsto \nu_x := \frac{1}{2} (\delta_{f(x)} + \delta_{-f(x)})$ from $[0,1]$ to probability measures on $\mathbb{R}$. If so, is there a problem in computing $W(\nu_x , \nu_y)$ for $x,y$ in $[0,1]$ ? $\endgroup$
    – user95282
    Commented May 16, 2017 at 0:42
  • $\begingroup$ I am confused because the $f$ in the definition of the kernel has nothing to do with the $f$ in the definition of the distance right? It follows: $\int g d\nu_x = \frac{1}{2}(g(f(x) + g(-f(x)))$? $\endgroup$
    – tubmaster
    Commented May 16, 2017 at 6:42
  • $\begingroup$ I don't think that the dilation $d$ is not a Lipschitz kernel for every discontinuous $f$. Take for example $f(x) = -1$, if $x \leq 0$ and $f(x) = 1$, if $x > 0$. Then $(x,0) \mapsto \frac{1}{2} \left( \delta_{(x,-1)} + \delta_{(x,1)} \right)$ is I think clearly Lipschitz (same as $x \mapsto \delta_x$). Or is $f$ is supposed to be nowhere continuous? But even then, we can take $f(x) = 1$ for $x\in \mathbb{Q}$ and $f(x) = -1$, else, which results in the same dilation. $\endgroup$
    – Steve
    Commented May 23, 2017 at 18:08

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