Optimal isoperimetric inequalities for surfaces in any codimension in Cartan-Hadamard manifolds

Let $(M^n,g)$ be simply connected, complete, with non-positive sectional curvatures, and $Σ$ a 2-dimensional closed integral current (or flat chain mod 2) with compact support in $M$. Let $S$ be an area minimising integral 3-current (resp. flat chain mod 2) such that $\partial S = Σ$. We use a weak mean curvature flow, obtained via elliptic regularisation, starting from $Σ$, to show that S satisfies the optimal Euclidean isoperimetric inequality: $6 \sqrtπ\, \mathbf{M}[S] \leq (\mathbf{M}[Σ])^{3/2}$. We also obtain an optimal estimate in case the sectional curvatures of M are bounded from above by $-κ< 0$ and characterise the case of equality. The proof follows from an almost monotonicity of a suitable isoperimetric difference along the approximating flows in one dimension higher and an optimal estimate for the Willmore energy of a 2-dimensional integral varifold with first variation summable in $L^2$.

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