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Let $X=(X_t)_{t\in I}$ ($I\subset\mathbb{R}$ a (non-degenerate) interval) be a stochastic process with continuous sample paths and such that $X_t$ admits a continuous Lebesgue density $\chi_t\in C(\mathbb{R}^d)$ for each $t\in I$.

Notation: Given $\delta>0$ and $t_0\in I$, we write $I_\delta(t_0):= I\, \cap \,(t_0-\delta, t_0+\delta)$.

Are you aware of (minimal) sufficient conditions on $X$ which guarantee that for each $K\subset\mathbb{R}^d$ compact the following implication holds:

$$\tag{$\star$}\min_{x\in K}\chi_{t_0}(x) > 0 \quad(\text{some }t_0\in I)\ \quad \Longrightarrow \quad \exists\,\delta>0 \, : \, \inf_{t\in I_\delta(t_0)}\,\min_{x\in K}\chi_{t}(x) > 0.$$

(A sufficient condition for $(\star)$ would be that $(t,x)\mapsto\chi_t(x)$ is continuous, but it is not clear how this translates to a natural condition on $X$.)

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    $\begingroup$ The answer at mathoverflow.net/questions/370159/… and discussion there show that your desired condition may not hold even when $X_t$ is however smooth in $t$, pathwise and on an average. So, I think no essentially non-tautological condition will suffice here. $\endgroup$ Commented Aug 26, 2020 at 15:07
  • $\begingroup$ @IosifPinelis Sorry to bother you with this extremely ill-defined question, but would you "feel" from your experience that imposing a given process $X$ to satisfy all of the above is a very restrictive model assumption if $X$ were to model a "generic data stream" (say ECG data or stock prices)? (The continuity of the density would be satisfied at least for some "reasonably-sized" class of time-homogeneous diffusions with continuously distributed initial value.) $\endgroup$
    – fsp-b
    Commented Aug 27, 2020 at 21:13
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    $\begingroup$ I think diffusions like this should cause no problems. However, as the example at mathoverflow.net/questions/370159/… shows, distributions can be however close to one another without any closeness of the corresponding densities. So, without any experience in such modeling, I'd guess it's better to avoid using broad classes of densities in modeling, and instead use integral characteristics such as cdf's and/or moments. $\endgroup$ Commented Aug 28, 2020 at 2:23
  • $\begingroup$ Thank you, @IosifPinelis. $\endgroup$
    – fsp-b
    Commented Aug 28, 2020 at 11:22

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