We present a 3D capsule architecture for processing of point clouds that is equivariant with respect to the SO(3) rotation group, translation and permutation of the unordered input sets. The network operates on a sparse set of local reference frames, computed from an input point cloud and establishes end-to-end equivariance through a novel 3D quaternion group capsule layer, including an equivariant dynamic routing procedure. The capsule layer enables us to disentangle geometry from pose, paving the way for more informative descriptions and a structured latent space. In the process, we theoretically connect the process of dynamic routing between capsules to the well-known Weiszfeld algorithm, a scheme for solving iterative re-weighted least squares (IRLS) problems with provable convergence properties, enabling robust pose estimation between capsule layers. Due to the sparse equivariant quaternion capsules, our architecture allows joint object classification and orientation estimation, which we validate empirically on common benchmark datasets.
This work builds a capsule network, for use with point cloud data, that has units that are equivariant to SO(3) 3D rotations.  I found that the authors made the case for rotation equivariance well and I liked the analysis of the dynamic routing approach and its mapping to the Generalized Weiszfeld Iterations. 
As a caveat to the review I should point out that while I am familiar with capsule networks, they are not my main area of expertise (I do work on geometry however) so this should be taken into consideration - apologies if there is literature I have missed. 
My main concern is that group equivariance for Capsule networks has been studied before - I acknowledge that SO(3) can present additional challenges but would have expected a work that specifically targeted SO(3) to investigate the relative benefits of the different approaches to encoding the lie algebra and not just quaternions. How do we know that quaternions are the best fit for the Capsule framework? Could the authors show empirical results that indicate that other representations for the equivariance do not work as well? My experience from optimization is that the best representation can often be application specific and it would be very helpful to understand their relative merits for this sort of architecture.
The preparation of the local patches and the estimation of the rotations seems very important to these applications to me and would be worthy of more discussion and an empirical presentation of their different merits. I know the authors discussed the issue of the second axis but it would seem worth including an experiment to detect the sensitivity in a similar manner to the point cloud density.
My biggest concern about the paper is the presentation of the results. Please could the authors provide appropriate error bars for the tables - this is the least action necessary to at least start to estimate significance. Ideally something like histograms or violin plots would be more useful. The use of a single dataset makes it hard to identify the efficacy of the approach - especially when it is not clear how the training and test data are set-up. Are the same objects used in training and test but with different rotations? Or different objects from the same categories. What is the protocol for the AR test set?
I agree that table 1 confirms the equivariance so that is great but it is less clear that the network is suitable for the pose task since the baselines seem very simple for the pose estimation experiment (and the related work covers a lot of prior work in this area) and again, without error bars it is hard to judge significance.
It is nice that the authors discuss limitations and I agree with the comment on symmetries - would it be possible to include an illustrative example to confirm these suspicions?
Other notes:
I have no issue with the proofs - they all seemed to make sense to me.
I found Figure 1 (and to a lesser extent Figure 3) to be unclear and uninformative - the notation doesn't match the main part of the paper (also where does C come from in the caption?) and its not clear what the illustrations are meant to represent. Would it not be much more clear to include something like Algorithm 1 instead of Figure 1 which is much more clear and easy to replicate the precise process.
I think there is a lot of material in the appendix that should really be in the main paper - I'm happy for the proofs to be in the appendix but I think it is a bit wrong to essentially violate the page restrictions by moving important related work into the appendix. The whole of section 2 is standard text-book information about quaternions - would it not be more appropriate for that to be in the appendix and the related work to be in the main paper?
Could the authors indicate the use of the Frobenius norm in eqn (5)? There also seems to be a mixing of notations for inner products through-out the paper and it might be helpful to standardize this?