Wulff Isoperimetry on Cayley Graphs: Submodular BV, Tempered Følner, and Profile Ratio Bounds

Abstract

We develop a BV framework on Cayley graphs, which yields a sharp discrete Wulff isoperimetric inequality with the best constant tied to the generating set/stencil $S$, a quantitative $Γ$-convergence of discrete to continuum anisotropic perimeter, and a gauge construction in Heisenberg group that neutralizes shear, yielding an BV+shear identity and scale-sharp compactness. As an application we revisit a question of Gromov (2008) on the ratio between the isoperimetric profile and its greatest nondecreasing minorant. We show that this is uniformly bounded on non-amenable and two-ended groups, and our Wulff inequality and $Γ$-convergence give a constant-tracked proof for virtually nilpotent groups. We isolate a Tempered Følner criterion (TF) (exhaustion principle with controlled increment), which forces bounded ratio in general. We verify (TF) in two families: finite-lamp wreath products over (TF) bases and lamplighters over amenable bases. For semidirect products $\mathbb Z^d\rtimes_A\mathbb Z$ we construct layer-nested sets of logarithmic height that are $A$-covariantly nested, Følner, and satisfy (TF)(ii) with a constant independent of $A\in\mathrm{GL}(d,\mathbb Z)$; and when $A$ is hyperbolic (no eigenvalue on the unit circle) we have full (TF). We also formulate ``gap conjectures'' that would settle the question for all amenable Cayley graphs. The BV viewpoint has spectral and analytic consequences: we derive constant-tracked Cheeger-type, Faber-Krahn, Nash inequalities, etc. The (TF) control further yields a tempered Property A, leading to explicit coarse embeddings into Hilbert space with compression $ρ(t) \gtrsim t^{1/2}/\log t$. Finally, we show that (TF) is a robust reflection of bi-equivariant geometry, and in virtually nilpotent classes this \emph{doubles} the sharp Wulff constant asymptotically-refining and answering another question of Gromov.