Flow Decomposition, Green Testing, and Lane--Emden Inequalities on Weighted Graphs

Abstract

We study positive solutions of the superlinear Lane--Emden inequality \[ -Δu\ge σu^q,\qquad q>1, \] on infinite locally finite weighted graphs and connected domains. When the Dirichlet Green function is finite, the existence of a positive solution is equivalent to \[ G_Ω\bigl(σg_Ω(o,\cdot)^q\bigr)(x) \le C g_Ω(o,x) \] for some pole \(o\inΩ\). Under Green function estimates, this yields sharp existence criteria and the Serrin-type exponents on \(\mathbb Z^d\) and orthant domains. For nonexistence, the principal method is flow decomposition. Its basic estimate bounds Green energy from below in terms of the relative capacities of intrinsic balls. %It yields annular conductance, capacity-to-infinity, and Nash--Williams cut-resistance criteria. For \(σ>0\), set \(ν=σμ\). We show that if \(d_ρ\) is a complete \(ν\)-adapted path metric and \[ \int_1^\infty \frac{r^{2q-1}}{ν(B_{d_ρ}(o,r))^{q-1}}\,dr=\infty, \] then every nonnegative solution is identically zero. The proof combines a flow decomposition of the acyclic Green current, a pathwise Hardy estimate, and a relative capacity estimate. It requires none of (VD), (PI), (P${}_0$), or the (3G) condition.