Wasserstein-pp local-dependence bound for normal approximation

Let pp be a positive integer and let W=iIXiW=\sum_{i\in I}X_i, where \mathbbmEXi=0{\mathbbm{E}}X_i=0, \mathbbmEW2=1{\mathbbm{E}}W^2=1, and the variables satisfy (LD1)--(LD(p+1)(p+1)): for each i1Ii_1\in I, i2Ai1,,imAi1im1i_2\in A_{i_1},\ldots,i_m\in A_{i_1\ldots i_{m-1}}, there is a set Ai1imAi1im1A_{i_1\ldots i_m}\supset A_{i_1\ldots i_{m-1}} such that Xi1,,Xim\\{X_{i_1},\ldots,X_{i_m}\\} is independent of Xj:jAi1im\\{X_j:j\notin A_{i_1\ldots i_m}}. Define

Rm=i1Ii2Ai1im+2Ai1im+1(\mathbbmE)\mathbbmEXi1Xi2(\mathbbmE)Xi3(\mathbbmE)Xim+2,R_m=\sum_{i_1\in I}\sum_{i_2\in A_{i_1}}\dots\sum_{i_{m+2}\in A_{i_1\dots i_{m+1}}}\sum_{({\mathbbm{E}})}{\mathbbm{E}}|X_{i_1}X_{i_2}|({\mathbbm{E}})|X_{i_3}|\cdots({\mathbbm{E}})|X_{i_{m+2}}|,

where (\mathbbmE)\sum_{({\mathbbm{E}})} is the sum over the possible placements of \mathbbmE{\mathbbm{E}} before each XiX_i, with any two \mathbbmE{\mathbbm{E}}'s separated by at least two XiX_i's. Wasserstein-pp conjecture. Under these assumptions,

Wp(L(W),n(0,1))Cpm=1p(Rm)1/m,\mathcal{W}_p(\mathcal{L}(W),n(0,1))\leqslant C_p\sum_{m=1}^p(R_m)^{1/m},

where CpC_p depends only on pp. This conjectured extension of the Wasserstein-1 normal-approximation bound concerns sums of locally dependent random variables. The case p=1p=1 was proved by Barbour, Karoński and Ruciński in 1989; the general assertion is therefore resolved at least in the stated base case, while the supplied evidence does not establish the status for every positive integer pp.

Sources & referencesView supporting material

Primary source

Xiao Fang, “Wasserstein-2 bounds in normal approximation under local dependence”, arXiv:1807.05741 (2019).

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