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The K User Linear Computation Broadcast (LCBC) problem is comprised of d dimensional data (from Fq), that is fully available to a central server, and K users, who require various linear computations of the data, and have prior knowledge of various linear functions of the data as side-information. The optimal broadcast cost is the minimum number of q-ary symbols to be broadcast by the server per computation instance, for every user to retrieve its desired computation. The reciprocal of the optimal broadcast cost is called the capacity. The main contribution of this paper is the exact capacity characterization for the K = 3 user LCBC for all cases, i.e., for arbitrary finite fields Fq, arbitrary data dimension d, and arbitrary linear side-informations and demands at each user. A remarkable aspect of the converse (impossibility result) is that unlike the 2 user LCBC whose capacity was determined previously, the entropic formulation (where the entropies of demands and side-informations are specified, but not their functional forms) is insufficient to obtain a tight converse for the 3 user LCBC. Instead, the converse exploits functional submodularity. Notable aspects of achievability include sufficiency of vector linear coding schemes, subspace decompositions that parallel those found previously by Yao Wang in degrees of freedom (DoF) studies of wireless broadcast networks, and efficiency tradeoffs that lead to a constrained waterfilling solution. Random coding arguments are invoked to resolve compatibility issues that arise as each user has a different view of the subspace decomposition, conditioned on its own side-information. 

Linear computation broadcast (LCBC) refers to a setting with d dimensional data stored at a central server, where K users, each with some prior linear side-information, wish to compute various linear combinations of the data. For each computation instance, the data is represented as a d-dimensional vector with elements in a finite field Fpn where pn is a power of a prime. The computation is to be performed many times, and the goal is to determine the minimum amount of information per computation instance that must be broadcast to satisfy all the users. The reciprocal of the optimal broadcast cost per computation instance is the capacity of LCBC. The capacity is known for up to K = 3 users. Since LCBC includes index coding as a special case, large K settings of LCBC are at least as hard as the index coding problem. As such the general LCBC problem is beyond our reach and we do not pursue it. Instead of the general setting (all cases), by focusing on the generic setting (almost all cases) this work shows that the generic capacity of the symmetric LCBC (where every user has m′ dimensions of side-information and m dimensions of demand) for large number of users (K ≥ d suffices) is Cg = 1/∆g, where ∆g = min{ max{0, d − m' }, dm/(m+m′)}, is the broadcast cost that is both achievable and unbeatable asymptotically almost surely for large n, among all LCBC instances with the given parameters p, K, d, m, m′. Relative to baseline schemes of random coding or separate transmissions, Cg shows an extremal gain by a factor of K as a function of number of users, and by a factor of ≈ d/4 as a function of data dimensions, when optimized over remaining parameters. For arbitrary number of users, the generic capacity of the symmetric LCBC is characterized within a factor of 2. 

Introduced by Sadeh et al., the K-star-graph private information retrieval (PIR) problem, so-labeled because the storage graph is a star-graph with K leaf nodes, is comprised of K messages that are stored separately (one-each) at K dedicated servers, and a universal server that stores all K messages, for a total of K + 1 servers. While it is one of the simplest PIR settings to describe, the capacity CK of K-star-graph PIR is open for K ≥ 4. We study the critical K = 4 setting, for which prior work establishes the bounds 2/5 ≤ C4 ≤ 3/7. As our main contribution, we characterize the exact capacity of 4-star-graph PIR as C4 = 5/12, thus improving upon both the prior lower- bound as well as the prior upper-bound. The main technical challenge resides in the new converse bound, whose non-trivial structure is deduced indirectly from the achievable schemes that emerge from the study of a finer tradeoff between the download costs from the dedicated servers versus the universal server. A sharp characterization of this tradeoff is also obtained for K = 4.