= Logarithmic approximation bound for slab-constrained quadratic maximization
{title2=$p_{\rm SDP}/[2\log(2m)]\le v^*\le p_{\rm SDP}$}
For spanning slab normals, apply the <maximum of finitely many Rademacher linear forms> bound to $u_i=(V\Lambda^{1/2})^Ta_i$, whose squared norms are $a_i^TXa_i\le1$. Some sign vector has scaling denominator squared at most $2\log(2m)$. <Rademacher rounding for a semidefinite relaxation> then produces a feasible vector of squared norm at least the displayed fraction of the SDP optimum. This is an existence guarantee from the <probabilistic method>.
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