The nonzero gain coefficients of Sobol's sequences are always powers of two
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When a plain Monte Carlo estimate on n samples has variance σ^2/n, then scrambled digital nets attain a variance that is o(1/n) as n→∞. For finite n and an adversarially selected integrand, the variance of a scrambled (t,m,s)-net can be at most Γσ^2/n for a maximal gain coefficient Γ<∞. The most widely used digital nets and sequences are those of Sobol'. It was previously known that Γ⩽ 2^t3^s for Sobol' points as well as Niederreiter-Xing points. In this paper we study nets in base 2. We show that Γ⩽2^t+s-1 for nets. This bound is a simple, but apparently unnoticed, consequence of a microstructure analysis in Niederreiter and Pirsic (2001). We obtain a sharper bound that is smaller than this for some digital nets. We also show that all nonzero gain coefficients must be powers of two. A consequence of this latter fact is a simplified algorithm for computing gain coefficients of nets in base 2.
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