Lower Bounds for Time-space Tradeoffs, Data Structures, and Proof Complexity

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This research addresses several problems in three areas of computational complexity. The overall objective in each case is to understand the limits of the capabilities of computers and the algorithms that run on them. The first area of research concerns tradeoffs between the processing time and storage requirements of computations: When can one solve problems using algorithms that are both fast and use small amounts of storage and when is one limited to trading off thesetwo resources, reducing storage requirements only at the cost of additional running time?The second area of research concerns the increased efficiencies in data structures that are possible by taking advantage of the wide range of instructions in modern processors that operate on entire computer words at once.The third area of research involves the analysis of algorithms to solve NP-hard search problems.When many such algorithms search but do not find a solution to a problem,they implicitly provide proofs that no such solution exists.In this portion of the research, the investigators analyze the form of these implicit proofs to show the limits of the efficiencies of these search algorithms.More specifically, the research in time-space tradeoffs builds on the investigators' previous work on time-space tradeoff lower bounds and is aimed at extending these results to other natural problems such as graph connectivity and improving the complexity limits shown by current lower bound techniques.The research in data structures works in the model of random-access machines with arbitrary unit-cost operations on word-size quantities.The goal of this data structure research is to understand the limits of thealgorithmic improvements possible in this model, particularly for nearest-neighbor queries, priority queues, sorting, and garbage collection.The research on the complexity of NP-search algorithms focuses onthe proof complexity of co-NP properties of random structures.The goal is to show that for randomly-chosen inputs, natural proof systems such as resolution almost always require exponential-size proofs of membership in the co-NP sets corresponding to the duals of natural NP problems.A consequence of this would be proofs that large classes of algorithms for solving the NP problems require exponential time on large numbers of inputs.