AF:Small: Applications of AP-free sets and derandomization

University Of Wisconsin-Madison, Madison WI

Investigators

Abstract

This project falls within the discipline of computational complexity, which studies the power and limitations of efficient computation. The area develops models that represent the various capabilities of digital computing devices. It aims to determine which transformations can be realized in such a way that the amount of time, memory space, and other resources scale moderately with the input size. Of central importance is the class of so-called NP-complete problems. The latter contains thousands of computational problems from all branches of science and engineering that have been shown equivalent in the sense that an efficient algorithm for one implies such an algorithm for all. The P vs NP question asks whether efficient algorithms exist for these problems. It constitutes the main open question in theory of computing and is one of the seven millennium prize problems proposed by the Clay Mathematics Institute as grand challenges for the 21st century. A positive answer would open up tremendous possibilities that would affect most human endeavors. On the other hand, it would also yield a way to break the cryptographic systems that are currently in use and, in fact, imply the impossibility of secure communication over the internet. This project fits into the quest to settle that fundamental and important problem. In particular, it establishes a tight connection between that question and the amount by which instances of NP-complete problems can be efficiently compressed without affecting their solvability. If P=NP, then NP-complete decision problems can be efficiently compressed to a single bit. On the other hand, under a hypothesis that is somewhat stronger than P<>NP, the PI has established that NP-complete problems like satisfiability and vertex cover do not allow any nontrivial amount of compression. The approach hinges on the existence of high-density subsets of the integers without arithmetic progressions of length 3. This project further develops that approach and investigates its implications for other computational parameters of interest. The project also involves a systematic study of the use of high-density subsets of the integers without arithmetic progressions of certain lengths in computational complexity, and the development of new applications. The above construction handles deterministic compression schemes. For cryptographic and other reasons the extension to the randomized setting is of interest. One possible approach involves derandomization. In this context the project investigates the potential of typically-correct derandomization, where one aims for efficient deterministic simulations that behave correctly on most but not necessarily all inputs of any given length.