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Nov 11, 2015 at 15:58 comment added Serguei Popov Seems this question was answered here: mathoverflow.net/questions/171527/… (although I'm not totally sure)
Nov 11, 2015 at 15:55 comment added Serguei Popov @Michael - but we need that $f_k$'s are i.i.d. That, is, $c_k$ shouldn't depend on $k$ then.
Nov 11, 2015 at 15:50 comment added Michael I should have said $c_k$ constants, which I would say have sub-exponential tails, regardless of how big they may be. But if you accept that you can't expect $X_n$ to be order of $\sqrt n$.
Nov 11, 2015 at 15:39 comment added Serguei Popov @Michael - yes, if $c_k$ are i.i.d.r.v. with exponential tails.
Nov 11, 2015 at 15:38 comment added Serguei Popov @Nate Eldredge - not much. $\sum M_k$ just grows linearly with positive speed, and we need to obtain that $\sum f_k$ somehow resembles a sum of 0-mean r.v.'s
Nov 11, 2015 at 15:37 history edited Serguei Popov CC BY-SA 3.0
added 13 characters in body
Nov 11, 2015 at 15:17 comment added Michael Let $Z_i$ be i.i.d. $\pm 1$ random variables. Do $f_k = c_kZ_k$ satisfy your hypotheses ?
Nov 11, 2015 at 15:08 comment added Nate Eldredge What do you get from the trivial bound $X_n \le \sum_{k=1}^n M_k$?
Nov 11, 2015 at 14:59 history asked Serguei Popov CC BY-SA 3.0