Lossless propagation bounds for long-range fermionic interactions
We prove propagation estimates for extensive fermionic many-body interactions with polynomial time growth, no loss of spatial decay, and an optimal threshold for the polynomial spatial exponent. More precisely, we construct local representatives of evolved fermionic many-body interactions on metric spaces with polynomial volume growth of order $d_{\mathrm{vol}}$ and suitable cell partitions. For the even Hamiltonian interaction $Φ_u$ and the even interaction $Ψ$ being evolved, we assume bounds, uniform in the site, on sums of term norms over finite supports $X$ containing a given site, with weights $e^{κ|X|}(1+\operatorname{diam}X)^a$ and $e^{κ|X|}(1+\operatorname{diam}X)^p$, respectively. Here $a>d_{\mathrm{vol}}$, $0\le p\le a$, $κ$ is sufficiently large, and the Hamiltonian bound is uniform in time. We construct local representatives whose interaction norms with weight $e^{c(1+|t-s|)^{-b}|X|}(1+\operatorname{diam}X)^p$ grow at most polynomially in $|t-s|$, for suitable $c,b>0$. In particular, $p=a$ is allowed: the Hamiltonian need not decay faster than the interaction being evolved. The threshold $a>d_{\mathrm{vol}}$ is optimal in general: uniform polynomial time bounds at $p=a$ fail for $0<a\le d_{\mathrm{vol}}$, even for smooth time-dependent two-site interactions. Polynomial time growth permits integration against filters with sufficiently many finite absolute moments, including those used in quasi-adiabatic constructions and quantum belief propagation. We derive consequences for filtered interactions and the dynamics they generate, providing dynamical estimates for perturbation theory with extensive perturbations, including gap stability, the principle that local perturbations perturb locally (LPPL), adiabatic theory, and response theory. All results also hold for quantum spin systems.