# News for July 2015

The summer month of July brings to us exciting work on distribution testing, scale-free graph testing, personal PageRank estimation, and (hold your breath!) cake cutting. Let’s hear more…

Avid followers of PTReview need little introduction to distribution testing, since we’ve seen much work on this topic over the past year. We have two concurrent results that really push the state-of-the-art of distribution testing.
Testing Shape Restrictions of Discrete Distributions by Clément Canonne, Ilias Diakonikolas, Themis Gouleakis, and Ronitt Rubinfeld (arxiv). Most (sublinear) distribution testing results work for structured classes, like monotone, k-modal, Poisson, multinomial etc. distributions. This paper gives an overarching framework that unifies these concepts in terms of succinctness. Consider a class $$\mathbb{C}$$ of distributions over $$[n]$$. This class is succinct if for any distribution $$\mathcal{D} \in \mathbb{C}$$, one can partition $$[n]$$ into a “small” number of intervals such that $$\mathcal{D}$$ is “approximately constant” with each interval. The main punchline is that any such class has a sublinear sample distribution tester. Amazingly, the generic tester given is optimal (up to poly log factors and the distance parameter)! Much previous work is subsumed as a corollary of this main theorem, and the paper also gives a lower bound framework to get matching lower bounds.

Optimal Testing for Properties of Distributions by Jayadev Acharya, Constantinos Daskalakis, and Gautam Kamath (arxiv). This paper concurrently gets many of the results of the previous paper using completely different techniques. The focus is on monotone, product, and log-concave distributions, where the paper gives completely optimal testers (both in terms on the domain size and the distance parameter). The key insight is to focus on the $$\chi^2$$-distance between distributions, though distribution testing typically uses the total-variation (or $$\ell_1$$) distance. The tester is also robust/tolerant, in that yes instances include distributions close to the class considered.

Bidirectional PageRank Estimation: From Average-Case to Worst-Case by Peter Lofgren, Siddhartha Banerjee, and Ashish Goel (arxiv). Not your typical sublinear algorithms paper, but that never stopped us. Personalized PageRank (PPR) is a fundamental similarity notion between vertices in a graph, and is (sort of) the probability that a short random walk from $$u$$ ends at $$v$$. Naturally, estimating the PPR value from $$u$$ to $$v$$ can be done in time $$O(1/p)$$, where $$p$$ is the PPR value. (Simply take many random walks from $$u$$, and see how often you hit $$v$$.) Can you do sublinear in this value? The answer is yes, when the graph is undirected and has bounded degree. One can do this in $$O(1/\sqrt{p})$$ time, a significant improvement. It basically uses bidirectional search, by doing forward walks from $$u$$ and backward walks from $$v$$. These ideas were developed in earlier work of Lofgren et al, based on true-blooded sublinear graph algorithms for expander reconstruction by Kale, Peres, and Seshadhri, which are further based on truer-blooded property testing questions by Goldreich and Ron on expansion testing. The best thing, these algorithms work in practice!

Every Property is Testable on a Natural Class of Scale-Free Multigraphs – ver. 2 by Hiro Ito (arxiv). Alas, PTRreview missed ver. 1, which appeared in April. One of the weaknesses of property testing on sparse graphs is the restriction to bounded degree. This paper considers arbitrary sparse graphs, and focuses on graphs with a power-law degree distribution. There are classic results of Barabási and Albert showing the significance of such graphs (also called scale-free networks) in the real-world. The main result is that special classes of scale-free graphs defined by clustering properties are hyperfinite. This means that one can remove a small fraction of edges to split the graph into constant-sized pieces. An important result of Newman and Sohler showed (in the bounded degree case) that any property defined by a subset of hyperfinite graphs graphs is testable. This paper leverages that result to show that properties defined by clustered classes of scale-free graphs are also testable.

How to solve the cake-cutting problem in sublinear time – ver. 2 by Hiro Ito and Takahiro Ueda (arxiv). Again, ver. 1 appeared in April and was missed by us. Apologies! Cake cutting is a classic resource allocation problem. Consider a cake, as the unit interval $$[0,1]$$. There are $$n$$ players, each of whom has a different utility of the cake, each specified by a continuous, real function on $$[0,1]$$. We wish to give each player a slice (subinterval of $$[0,1]$$) such that they all have utility $$1/n$$. An old result of Even and Paz give $$O(n \log n)$$ divide and conquer algorithms. What can be said in sublinear time? This paper shows that one can give fair assignments to $$o(n)$$ players in $$o(n)$$ time, such that the assignment can be extended to almost all remaining players.