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1. Lower Bounds for Testing Complete Positivity and Quantum Separability

   https://doi.org/10.1007/978-3-030-61792-9_30  

   Bădescu, Costin; O'Donnell, Ryan (December 2020, LATIN 2020: Theoretical Informatics)

   Kohayakawa, Y.; Miyazawa, F.K. (Ed.)

   In this work we are interested in the problem of testing quantum entanglement. More specifically, we study the separability problem in quantum property testing, where one is given n copies of an unknown mixed quantum state ϱ on Cd⊗Cd , and one wants to test whether ϱ is separable or ϵ -far from all separable states in trace distance. We prove that n=Ω(d2/ϵ2) copies are necessary to test separability, assuming ϵ is not too small, viz. ϵ=Ω(1/d−−√) . We also study completely positive distributions on the grid [d]×[d] , as a classical analogue of separable states. We analogously prove that Ω(d/ϵ2) samples from an unknown distribution p are necessary to decide whether p is completely positive or ϵ -far from all completely positive distributions in total variation distance. 

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2. NP-Hardness of Reed-Solomon Decoding and the Prouhet-Tarry-Escott Problem

   https://doi.org/10.1109/FOCS.2016.86  

   Gandikota, Venkata; Ghazi, Badih; Grigorescu, Elena (October 2016, FOCS)

   Establishing the complexity of {\em Bounded Distance Decoding} for Reed-Solomon codes is a fundamental open problem in coding theory, explicitly asked by Guruswami and Vardy (IEEE Trans. Inf. Theory, 2005). The problem is motivated by the large current gap between the regime when it is NP-hard, and the regime when it is efficiently solvable (i.e., the Johnson radius). We show the first NP-hardness results for asymptotically smaller decoding radii than the maximum likelihood decoding radius of Guruswami and Vardy. Specifically, for Reed-Solomon codes of length $$N$$ and dimension $K=O(N)$, we show that it is NP-hard to decode more than $$ N-K- c\frac{\log N}{\log\log N}$$ errors (with $c>0$ an absolute constant). Moreover, we show that the problem is NP-hard under quasipolynomial-time reductions for an error amount $$> N-K- c\log{N}$$ (with $c>0$$ an absolute constant). An alternative natural reformulation of the Bounded Distance Decoding problem for Reed-Solomon codes is as a {\em Polynomial Reconstruction} problem. In this view, our results show that it is NP-hard to decide whether there exists a degree $$K$$ polynomial passing through $$K+ c\frac{\log N}{\log\log N}$$ points from a given set of points $$(a_1, b_1), (a_2, b_2)\ldots, (a_N, b_N)$$. Furthermore, it is NP-hard under quasipolynomial-time reductions to decide whether there is a degree $$K$$ polynomial passing through $$K+c\log{N}$$ many points. These results follow from the NP-hardness of a generalization of the classical Subset Sum problem to higher moments, called {\em Moments Subset Sum}, which has been a known open problem, and which may be of independent interest. We further reveal a strong connection with the well-studied Prouhet-Tarry-Escott problem in Number Theory, which turns out to capture a main barrier in extending our techniques. We believe the Prouhet-Tarry-Escott problem deserves further study in the theoretical computer science community. 

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3. Classification of Constructible Cosheaves

   Curry, J; Patel, A (January 2020, Theory and applications of categories)

   null (Ed.)

   In this paper we prove an equivalence theorem originally observed by Robert MacPherson. On one side of the equivalence is the category of cosheaves that are constructible with respect to a locally cone-like stratification. Our constructibility condition is new and only requires that certain inclusions of open sets are sent to isomorphisms. On the other side of the equivalence is the category of functors from the entrance path category, which has points for objects and certain homotopy classes of paths for morphisms. When our constructible cosheaves are valued in Set we prove an additional equivalence with the category of stratified coverings. 

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4. Classification of Constructible Cosheaves

   Curry, Justin; Patel, Amit (June 2020, Theory and applications of categories)

   In this paper we prove an equivalence theorem originally observed by Robert MacPherson. On one side of the equivalence is the category of cosheaves that are constructible with respect to a locally cone-like stratification. Our constructibility condition is new and only requires that certain inclusions of open sets are sent to isomorphisms. On the other side of the equivalence is the category of functors from the entrance path category, which has points for objects and certain homotopy classes of paths for morphisms. When our constructible cosheaves are valued in Set we prove an additional equivalence with the category of stratified coverings. 

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5. Even 1xn Edge-Matching and Jigsaw Puzzles are Really Hard

   Bosboom, J.; Demaine, E.D.; Demaine, M.L.; Hesterberg, A.; Manurangsi, P.; Yodpinyanee, A. (January 2017, Journal of information processing)

   We prove the computational intractability of rotating and placing n square tiles into a 1 × n array such that adjacent tiles are compatible—either equal edge colors, as in edge-matching puzzles, or matching tab/pocket shapes, as in jigsaw puzzles. Beyond basic NP-hardness, we prove that it is NP-hard even to approximately maximize the number of placed tiles (allowing blanks), while satisfying the compatibility constraint between nonblank tiles, within a factor of 0.9999999702. (On the other hand, there is an easy (1/2)-approximation.) This is the first (correct) proof of inapproximability for edge-matching and jigsaw puzzles. Along the way, we prove NP-hardness of distinguishing, for a directed graph on n nodes, between having a Hamiltonian path (length n − 1) and having at most 0.999999284 (n − 1) edges that form a vertex-disjoint union of paths. We use this gap hardness and gap-preserving reductions to establish similar gap hardness for 1 × n jigsaw and edge-matching puzzles. 

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