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ABSTRACT
We prove lower bounds on the redundancy necessary to represent a set S of objects using a number of bits close to the information-theoretic minimum log2 |S|, while answering various queries by probing few bits. Our main results are: To represent n ternary values t ∈ {0,1,2}n in terms of u bits b ∈ {0,1}u while accessing a single value ti ∈ {0,1,2} by probing q bits of b, one needs u ≥ (log2 3)n + n/2O(q). This matches an exciting representation by Patrascu (FOCS 2008), later refined with Thorup, where u ≤ (log_2 3)n + n/2Ω(q). We also note that results on logarithmic forms imply the lower bound u ≥ (log2 3)n + n/logO(1) n if we access ti by probing one cell of log n bits. To represent sets of size n/3 from a universe of n elements in terms of u bits b ∈ {0,1}u while answering membership queries by probing q bits of b, one needs u ≥ log2 n/(n/3) + n/2O(q) - log n. Both results above hold even if the probe locations are determined adaptively. Ours are the first lower bounds for these fundamental problems; we obtain them drawing on ideas used in lower bounds for locally decodable codes. REFERENCES
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