Equiangular tight frames (ETFs) are optimal packings of lines through the origin. At the moment, they are the subject of a rapidly growing literature. In fact, there have been quite a few updates since my last post on this subject (less than five months ago), and I’ve revamped the table of ETFs accordingly. What follows is a brief discussion of the various developments:
1. There is an ETF of 76 vectors in
See this paper. Last time, I mentioned a recent proof that there is no ETF of 76 vectors in . It turns out that a complex ETF of this size does exist. To prove this, it actually seems more natural to view the vectors as columns of a matrix whose row vectors sum to zero. As a lower-dimensional example, consider the following matrix:
Continue reading Recent developments in equiangular tight frames, II
This month, several experts in frame theory will be visiting my department, and so Matt Fickus and I decided to organize a workshop in the style of AIM. Considering the recent progress we’ve made on equiangular tight frames (ETFs) — namely, one, two, three, and four — we are hoping this workshop will spur further progress in this area. To kick off the month, I asked a few people to prepare hour-long chalk talks, and what follows are the extended abstracts:
1. Introduction to ETFs (Dustin G. Mixon)
Given a -dimensional Hilbert space space and a positive integer , we are interested in packing lines through the origin so that the interior angle between any two is as large as possible. It is convenient to represent each line by a unit vector that spans the line, and in doing so, the problem amounts to finding unit vectors that minimize coherence:
This minimization amounts to a nonconvex optimization problem. To construct provably optimal packings, one must prove a lower bound on for a given and spatial dimension , and then construct an ensemble which meets equality in that bound. To date, we know of three bounds that are sharp:
Continue reading SOFT 2016: Summer of Frame Theory
A couple of weeks ago, I attended a workshop hosted by Darrin Speegle on the HRT Conjecture. First posed twenty years ago by Chris Heil, Jay Ramanathan, and Pankaj Topiwala in this paper, the conjecture states that every finite collection of time-frequency shifts of any nonzero function in is linearly independent. In this post, I will discuss some of the key ideas behind some of the various attempts at chipping away at HRT, and I’ll describe what appear to be fundamental barriers to a complete solution. For more information, see these survey papers: one and two.
First, some notation: Denote the translation-by- and modulation-by- operators by
respectively. Then the formal conjecture statement is as follows:
The HRT Conjecture. For every and every finite , the collection is linearly independent.
What follows are some of the popular methods for tackling HRT:
Continue reading The HRT Conjecture
From MaxCut to PhaseLift, semidefinite programming has proven to be rather powerful, especially for convex relaxation. SDP solvers take polynomial time, but the exponent is large, and anyone who’s run an SDP on CVX has experienced some frustration with the runtime. In practice, the SDP-optimal matrix tends to have extremely low rank, and so one may apply a rank constraint to facilitate the search for the SDP’s solution. This heuristic was first introduced by Burer and Monteiro, and it works well in practice, but the rank-constrained program is nonconvex and the theory is scant. Recently, the theory gap started to close with this paper:
On the low-rank approach for semidefinite programs arising in synchronization and community detection
Afonso S. Bandeira, Nicolas Boumal, Vladislav Voroninski
As the title suggests, this paper provides strong performance guarantees for the Burer-Monteiro heuristic in the particular cases of synchronization and community detection. I was very excited to see this paper, and so I interviewed one of the authors (Nicolas Boumal). I’ve lightly edited his responses for formatting and hyperlinks:
Continue reading On the low-rank approach for semidefinite programs arising in synchronization and community detection
Soledad Villar recently posted our latest paper on the arXiv (this one coauthored by her advisor, Rachel Ward). The paper provides guarantees for the k-means SDP when the points are drawn from a subgaussian mixture model. This blog entry will discuss one of the main ideas in our analysis, which we borrowed from Guedon and Vershynin’s recent paper.
Let’s start with two motivating applications:
The first application comes from graph clustering. Consider the stochastic block model, in which the vertices are secretly partitioned into two communities, each of size , and edges between vertices of a common community are drawn iid with some probability , and all other edges are drawn with probability . The goal of community estimation is to estimate the communities given a random draw of the graph. For this task, you might be inclined to find the maximum likelihood estimator for this model, but this results in an integer program. Relaxing the program leads to a semidefinite program, and amazingly, this program is tight and recovers the true communities with high probability when and for good choices of . (See this paper.) These edge probabilities scale like the threshold for connected Erdos-Renyi graphs, and this makes sense since we wouldn’t know how to assign vertices in isolated components. If instead, the probabilities were to scale like , then we would be in the “giant component” regime, so we’d still expect enough signal to correctly assign a good fraction of the vertices, but the SDP is not tight in this regime.
Continue reading Clustering noisy data with semidefinite relaxations
Equiangular tight frames (ETFs) are optimal packings of lines through the origin. Last year, Matt and I tabulated all known existence results for ETFs. Since then, there have been a few interesting developments on the subject, so I’ll have to update the table soon. In the meantime, this blog entry covers the highlights.
First, some context: In the real case, ETFs are in one-to-one correspondence with certain strongly regular graphs (SRGs). Waldron’s paper on the subject is the standard modern treatment of this correspondence. SRGs have been an active area of research for a few decades, and Brouwer’s table provides a survey of the various known constructions. This suggests a straightforward program for constructing real ETFs: Go to Brouwer’s table, find an SRG of the requisite size, investigate the reference that constructs that graph, follow Waldron’s treatment to construct the corresponding Gram matrix, and then decompose the Gram matrix to get the desired ETF.
Now for the news:
Continue reading Recent developments in equiangular tight frames