Gaussian comparison conjecture for maxima under variance and increment domination

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Let X=(X1,…,Xd)\mathbf{X}=(X_1,\ldots,X_d) and Y=(Y1,…,Yd)\mathbf{Y}=(Y_1,\ldots,Y_d) be dd-dimensional Gaussian vectors with common mean μ=(μ1,…,μd)\bm{\mu}=(\mu_1,\ldots,\mu_d) and covariance matrices ΣX\Sigma^X and ΣY\Sigma^Y, whose ijij entries are σijX\sigma_{ij}^X and σijY\sigma_{ij}^Y, respectively. Define

γijX=E{(Xi−Xj)2},γijY=E{(Yi−Yj)2}.\gamma_{ij}^X=\mathbb{E}\{(X_i-X_j)^2\},\qquad \gamma_{ij}^Y=\mathbb{E}\{(Y_i-Y_j)^2\}.

Let med⁡(max⁡iYi)\operatorname{med}(\max_iY_i) denote the median of max⁡1≤i≤dYi\max_{1\leq i\leq d}Y_i, namely the value aa satisfying Pr⁡(max⁡1≤i≤dYi≤a)=0.5\Pr(\max_{1\leq i\leq d}Y_i\leq a)=0.5. Suppose that σiiY≥σiiX\sigma_{ii}^Y\geq\sigma_{ii}^X for every ii and γijY≥γijX\gamma_{ij}^Y\geq\gamma_{ij}^X for every i,ji,j. For any c≥med⁡(max⁡iYi)c\geq\operatorname{med}(\max_iY_i), Gaussian comparison conjecture. There is a comparison bound of the form

Pr⁡(max⁡1≤i≤dXi≥c)≤(?)  Pr⁡(max⁡1≤i≤dYi≥c).\Pr\left(\max_{1\leq i\leq d}X_i\geq c\right)\leq (?)\;\Pr\left(\max_{1\leq i\leq d}Y_i\geq c\right).

Under these assumptions, the Sudakov--Fernique inequality gives only the expectation comparison E{max⁡iXi}≤E{max⁡iYi}\mathbb{E}\{\max_iX_i\}\leq\mathbb{E}\{\max_iY_i\}; the conjectured tail comparison would yield the desired asymptotic sharp-dominance result for the multivariate one-sided Gaussian-prepivoted test. It is stated to be true in the univariate case, while the multivariate case remains open.

References

Primary source

Peter L. Cohen and Colin B. Fogarty, “Gaussian Prepivoting for Finite Population Causal Inference”, arXiv:2002.06654 (2021).

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