Hyperscaling for oriented percolation in 1+1 space-time dimensions

Consider nearest-neighbor oriented percolation in $d+1$ space-time dimensions. Let $\rho,\eta,\nu$ 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 $d\nu\ge\eta+2\rho$, which holds for all $d\ge1$ and is a strict inequality above the upper-critical dimension 4, becomes an equality for $d=1$, i.e., $\nu=\eta+2\rho$, provided existence of at least two among $\rho,\eta,\nu$. The key to the proof is the recent result on the critical box-crossing property by Duminil-Copin, Tassion and Teixeira (2017).