Fix $n\geq 2$ and let $$\mathbb{H}^{n}=\mathbb{R}\_{+}\times \mathbb{S}^{n-1}$$ be the hyperbolic space, so that any point $x\in \mathbb{H}^{n}$ can be represented in polar coordinates $x=(r, \theta)$, and equipped with the Riemannian metric $$g=dr^{2}+\sinh^{2}(r)d\theta^{2},$$ where $dr^{2}$ is the standard metric on $\mathbb{R}\_{+}$ and $d\theta^{2}$ is the standard metric on the sphere $\mathbb{S}^{n-1}$. Denote with $\mu$ the Riemannian measure on $\mathbb{H}^{n}$ and with $\sigma$ the Riemannian measure of co-dimension $1$ on hypersurfaces on $\mathbb{H}^{n}$. It is a known fact that $\mathbb{H}^{n}$ admits the Isoperimetric inequality $$\sigma(\partial \Omega)\geq f(\mu(\Omega)),$$ for all precompact open sets $\Omega\subset \mathbb{H}^{n}$ with smooth boundary, where $f$ is defined by $$f(v)=c\_{1}\left\{\begin{array}{cl}v, &v\geq 1\\ v^{\frac{n-1}{n}}, &v\leq 1,\end{array}\right.$$ for some small constant $c\_{1}>0$.


Let us fix some precompact open set $U\subset \mathbb{H}^{n}$ and consider $\mathbb{H}^{n}\setminus U$ as a manifold with boundary $\partial U$.


Now my **question**:



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> Does the same Isoperimetric inequality now hold for precompact open sets $\Omega\subset \mathbb{H}^{n}\setminus U$ with smooth boundary (and possibly a smaller constant $c\_{2}>0$) and if yes, where can I find a reference for this statement?
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> 


**Specified question**:



> 
> Let $U$ be a "nice" precompact open set, for example $U=B\_{1}(o)$ is an open ball of radius $1$ for some point $o\in \mathbb{H}^{n}$. Does the same Isoperimetric inequality now hold for precompact open sets $\Omega\subset \mathbb{H}^{n}\setminus B\_{1}(o)$ with smooth boundary (and possibly a smaller constant $c\_{2}>0$)?
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Note that we assume that, when considering a precompact open set $\Omega\subset \mathbb{H}^{n}\setminus U$ with smooth boundary, we have $\partial U\cap \partial \Omega=\emptyset$.


If this is not known for the hyperbolic space $\mathbb{H}^{n}$, is a similar statement known for other Riemannian manifolds, for example $\mathbb{R}^{n}$?


Thanks in advance for your help!



Isoperimetric inequality for domains in the exterior of a precompact open set in Riemannian manifold
Some results are known under some restrictions. First, no uniform inequality can hold if $U$ is unrestrained: it could take the shape of (a neighborhood of) a bottle, and the bottleneck will make it possible to have $\Omega$ with large volume and small boundary. A natural restriction is to consider convex $U$ (so that the double of $\mathbb{H}^n$ still is negatively curved in the metric sense).


**To sum up:** assuming $U$ is convex, we even have sharp bounds. When $M=\mathbb{H}^n$ the dimensions $n=2,3,4$ are covered; when $M=\mathbb{R}^n$, you have a weaker inequality (the isoperimetric function has the form $f(v) = cv^{\frac{n-1}n}$, there is no linear asymptotic) but known in all dimensions.


**Added in edit:** I think that an equality $\sigma(\partial \Omega)\ge c\mu(\Omega)$ is true in all dimensions whenever $U$ is convex, and that the method in our paper with Kuperberg mentionned below can be used to prove this. The more precise bound for small $v$, I am not sure.


Note that $\mathbb{H}^n$ can even be replaced by a simply connected manifold $M$ of sectional curvature bounded above by some $\kappa\le 0$. In this setting:


1. Choe proved in 2003 gave the *sharp* inequality when $M=\mathbb{R}^n$ and $U$ is a ball, as well as several other restricted cases: **Relative isoperimetric inequality for domains outside a convex set.** *Archives Inequalities Appl 1 (2003): 241-250*.
2. In 2007, the general case of a convex set $U\subset \mathbb{R}^n$ was obtained by Choe, Ghomi and Ritoré. **The relative isoperimetric inequality outside convex domains in $\mathbf{R}^n$.** *Calculus of Variations and Partial Differential Equations 29.4 (2007): 421-429.*
3. In 2006, Choe treated the case when $M$ has variable (nonpositive) curvature and dimension $4$. **The double cover relative to a convex domain and the relative isoperimetric inequality.** *Journal of the Australian Mathematical Society 80.3 (2006): 375-382.*
4. In dimension $3$, the same was achieved by Choe and Ritoré in 2007. **The relative isoperimetric inequality in Cartan-Hadamard 3-manifolds.** *J. für die reine und angewandte Mathematic (2007): 179-191.* They also obtain the sharp inequality when $\kappa=-1$, in particular for $M=\mathbb{H}^3$.
5. With Kuperberg, we gave a new proof of Choe's result from 2006, treating dimensions $2$ and $4$ and including (sharp) results when $M$ has curvature bounded above by $\kappa$ (either $0$ or negative). **The Cartan–Hadamard conjecture and the Little Prince.** *Revista Matemática Iberoamericana 35.4 (2019): 1195-1258.* Our results can be used to tackle some finite union of convex sets (you need to ensure that geodesic rays reflecting on the boundary of $U$ can only bounce a bounded number of time).


I may have missed other relevant references, but you should catch them by looking at papers citing the above ones.


