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Note from OP: I gave up and reposted this Question with a Bounty on Cross Validated HERE.

In certain applications, we approximate an unknown pdf by placing uniformly weighted Gaussian terms at each of some sample points $\{x_{1},...,x_{n}\} $ and assigning some variances $\{\sigma_{1},...,\sigma_{n}\} $:

$$f(x)\equiv\frac{1}{n}\sum_{i=1}^{n}N(x-x_{i},\sigma_{i})=\frac{1}{n}\sum_{i=1}^{n}\frac{1}{\sqrt{2\pi\sigma_{i}^{2}}}e^{-\frac{(x-x_{i})^{2}}{2\sigma_{i}^{2}}}$$

It seems intuitive that the sampled log-liklihood would be greater than (or equal to) the mean log-liklihood:

$$\frac{1}{n}\sum_{j=1}^{n}ln(f(x_{j}))\geq\int f(x)ln(f(x))dx $$

This is obviously true for small variances (each $x_{i}$ is on top of a narrow Gaussian) and for very large variances (all $x_{i}$'s are together atop one broad Gaussian), and it's true for every random set of $x_i$'s and $\sigma_i$'s variances I've tried, but I can't prove it's true for all sets of variances $\{\sigma_{1},...,\sigma_{n}\}$. I suspect Jensen's Inequality would be part of such a proof, but I'm stumped.

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  • $\begingroup$ Why would you use $n$ different Gaussians if you have $n$ data points? $\endgroup$ Commented Jun 18, 2016 at 5:22
  • $\begingroup$ @DouglasZare I know, it's a strange thing to do. The reasons are a bit too involved to discuss here. $\endgroup$ Commented Jun 18, 2016 at 5:36
  • $\begingroup$ If you are actually doing this as opposed to having a thought experiment about overfitting, I hope you are regularizing the heck out of the parameters. $\endgroup$ Commented Jun 18, 2016 at 5:58
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    $\begingroup$ This was cross-posted at Mathematics math.stackexchange.com/questions/1829100/… and at Cross Validated stats.stackexchange.com/questions/219459/…. Please don't do that. $\endgroup$ Commented Jun 18, 2016 at 10:18
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    $\begingroup$ Different sites have different target audiences. The best policy is to decide which site is best and then post there; if no response comes after a few days, then it's generally okay with us (not so much with SE management) if you want to post it here, provided that you link, at each site where it is posted, to all the posts at other sites. I would guess that Cross Validated would have been best. See also meta.stackexchange.com/questions/64068/… $\endgroup$ Commented Jun 18, 2016 at 10:33

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