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Let $T:=[-1,1]^{n-1}\times (0,1]$. Let

$$f_n(x_1,\cdots,x_n,w_1,\cdots,w_n):=g(x_1,w_1)+\cdots+g(x_n,w_n)=\sum_{i=1}^ng(x_i,w_i),$$ where

(i) $w_1,\cdots,w_n$ are i.i.d. Gaussian random variables

(ii) $g(x_i,w_i)$ is a smooth function ($g\in C^\infty$)

(iii) $\mathbb{E}g(x_i,w_i)>0$

(iv) $\inf_{(x_1,\cdots,x_n)\in T}\mathbb{E}(\sum_{i=1}^ng(x_i,w_i))=0$

My question: is there any difference between the following two?

(1) for any $(x_1,\cdots,x_n)\in T$, we have $$P(\sum_{i=1}^ng(x_i,w_i)>0)\rightarrow 1$$ when $n$ goes to infinity.

(2) $$P\big(\inf_{(x_1,\cdots,x_n)\in T}\sum_{i=1}^ng(x_i,w_i)>0\big)\rightarrow 1$$ when $n$ goes to infinity.

Thanks for any suggestion!

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  • 1
    $\begingroup$ What is $\vec x^*$? $\endgroup$ Commented May 28, 2023 at 1:49
  • $\begingroup$ "When $n$ goes to infinity"? So you have a whole family of functions $f$, one for each $n$? $\endgroup$ Commented May 28, 2023 at 7:28
  • $\begingroup$ @IosifPinelis sorry for the confusion, I re-edited my question. I hope this time is much clear. $\endgroup$
    – happyle
    Commented May 28, 2023 at 9:07
  • $\begingroup$ @GeraldEdgar yes, $f$ is a random function $X_{t\in T\subset R^n}$ and its dimension $n$ grows $\endgroup$
    – happyle
    Commented May 28, 2023 at 9:44
  • $\begingroup$ What could $X_{t\in T}$ possibly mean? $\endgroup$ Commented May 28, 2023 at 13:49

1 Answer 1

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A trivial counterexample: $$g(x,w):=x.$$

Then convergence (1) holds but convergence (2) does not.

On the other hand, of course, convergence (2) always implies convergence (1).

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  • $\begingroup$ thank you! Other than '$T$ is compact' (then $\inf=\min$), is there other condition that we could add such that (1) implies (2)? $\endgroup$
    – happyle
    Commented May 28, 2023 at 18:18
  • $\begingroup$ @happyle : You can see we have worked quite a bit to make the conditions sensible. But what can sensibly be added, I don't know. $\endgroup$ Commented May 28, 2023 at 18:31
  • $\begingroup$ thanks! this is already very helpful. $\endgroup$
    – happyle
    Commented May 28, 2023 at 18:35

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