## The Long Reach of Reachability

* Workshop on Infinite State Systems at the Bellairs Institute on Barbados *

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Joel Ouaknine is a Professor of Computer Science at Oxford University and a Fellow of St. John’s College there. He was previously a doctoral student at Oxford and made a critical contribution in 1998 of a kind I enjoyed as a student in the 1980s. This was contributing a win in the annual Oxford-Cambridge Varsity Chess Match, which in 1998 was won by Oxford, 5-3.

Today I’d like to report on some of the wonderful things that happened at a workshop on “Infinite-State Systems” hosted by Joel at the Bellairs Institute of McGill University last March 13–20 in Barbados, before we finally opened a chess set and played two games on the last evening.

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## The Halting Problem For Linear Machines

* A small idea before the fireworks show *

Thoralf Skolem was a mathematician who worked in mathematical logic, set theory, and number theory. He was the only known PhD student of Axel Thue, whose Thue systems were an early word-based model of computation. Skolem had only one PhD student, Öystein Ore, who did not work in logic or computation. Ore did, however, have many students including Grace Hopper and Marshall Hall, Jr., and Hall had many more including Don Knuth.

Today Ken and I try to stimulate progress on a special case of Skolem’s problem on linear sequences.

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## We Say Branches And You Say Choices

* You like tomato and I like tomahto *

Oded Green, Marat Dukhan, and Richard Vuduc are researchers at Georgia Institute of Technology—my home institution. They recently presented a paper at the Federated Conference titled, “Branch-Avoiding Graph Algorithms.”

Today Ken and I would like to discuss their interesting paper, and connect it to quite deep work that arises in computational logic.

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## Fathering Randomized Fault Tolerance

* Plus visiting Michael Rabin and talking about Gödel’s Theorems *

Michael Ben-Or and Michael Rabin have won the 2015 Dijkstra Prize for Distributed Computing. The citation says,

In [two] seminal papers, published in close succession in 1983, Michael Ben-Or and Michael O. Rabin started the field of fault-tolerant randomized distributed algorithms.

Today Ken and I wish to congratulate both Michaels for the well deserved recognition for their brilliant work.

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## Security Via Surrender

* A new approach to protecting data and identity *

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David Sanger and Julie Davis are reporters for the paper of record—the New York Times. Their recent article starts:

WASHINGTON—The Obama administration on Thursday announced what appeared to be one of the largest breaches of federal employees’ data, involving at least four million current and former government workers in an intrusion that officials said apparently originated in China.

The compromised data was held by the Office of Personnel Management, which handles government security clearances and federal employee records. The breach was first detected in April, the office said, but it appears to have begun at least late last year.

The target appeared to be Social Security numbers and other “personal identifying information,” but it was unclear whether the attack was related to commercial gain or espionage. …

Today Ken and I want to suggest a new approach to data breaches like this.

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## Minor Insights Are Useful

* Some examples of small insights that help *

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Julia Chuzhoy and Chandra Chekuri are experts on approximation algorithms: both upper and lower bounds. Each is also interested in graph theory as it applies to algorithms.

Today Ken and I wish to talk about their recent papers on structural theorems for graphs.

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## Puzzling Evidence

* Exponential hardness connects broadly to quadratic time *

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Arturs Backurs and Piotr Indyk are student and advisor. The latter, Piotr, is one of the leading algorithms and complexity theorists in the world—what an honor it must be for Arturs to work with him as his advisor.

Today Ken and I want to talk about their paper on edit distance and an aspect that we find puzzling.

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