Consider nearest-neighbor oriented percolation in $d+1$ space-time dimensions. Let $rho,eta, u$ be the critical exponents for the survival probability up to time $t$, the expected number of vertices at time $t$ connected from the space-time origin, and the gyration radius of those vertices, respectively. We prove that the hyperscaling inequality $d ugeeta+2rho$, which holds for all $dge1$ and is a strict inequality above the upper-critical dimension 4, becomes an equality for $d=1$, i.e., $ u=eta+2rho$, provided existence of at least two among $rho,eta, u$. The key to the proof is the recent result on the critical box-crossing property by Duminil-Copin, Tassion and Teixeira (2017).
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