# Gromov–Hausdorff Distances

The Gromov-Hausdorff (d_{GH}) distance provides a reasonable framework for comparing shapes. Though the Gromov-Hausdorff distances between metric spaces are a common concept in geometry and in data analysis, these distances are hard to compute. Although the authors of showed the \mathcal{NP}-hardness of approximation of the GH distance between metric trees, computing the distance between Euclidean subsets is still elusive. To address such questions in the Euclidean space, I started a collaborative project with Carola Wenk and Jeffrey Vitter. In our effort to approximate the GH distance for subsets of \mathbb{R}^d, I studied its relationship with d_{H,iso}, the minimum Hausdorff distance under the class of Euclidean isometries. For d=1, we showed in~ that d_{H,iso}\leq\left(1+\frac{1}{4}\right)d_{GH}. This gives rise to an O(n\log{n})-time algorithm for approximating d_{GH} with an approximation factor of \left(1+\frac{1}{4}\right).

I believe that computing the distance is \mathcal{NP}-hard for Euclidean subsets. I steer the current investigation towards such hardness questions, along with the question of approximating d_{GH} for d>1. At the same time, I conjecture that the exact computation is polynomial-time for d=1.