In limit theorems, one of the biggest problem is to give an answer to Ibragimov's conjecture, which states the following:
Let (Xn,ninBbbN) be a strictly stationary phi-mixing sequence, for which mathbbE(X20)<infty and operatornameVar(Sn)to+infty. Then Sn:=sumnj=1Xj is asymptotically normally distributed.
phi-mixing coefficents are defined as
phiX(n):=sup(|mu(BmidA)−mu(B)|,AinmathcalFm,BinmathcalFm+n,minBbbN),
where mathcalFm and mathcalFm+n are the sigma-algebras generated by the Xj, jleqslantm (respectively jgeqslantm+n), and phi-mixing means that phiX(n)to0.
It was posed in Ibragimov and Linnik paper in 1965.
Peligrad showed the result holds with the assumption liminfnto+inftyn−1operatornameVar(Sn)>0. It also holds when mathbbElvertX0rvert2+delta is finite for some positive delta (Ibragimov, I think).
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