Theory of Computing ------------------- Title : A Characterization of Hard-to-Cover CSPs Authors : Amey Bhangale, Prahladh Harsha, and Girish Varma Volume : 16 Number : 16 Pages : 1-30 URL : https://theoryofcomputing.org/articles/v016a016 Abstract -------- We continue the study of the _covering complexity_ of constraint satisfaction problems (CSPs) initiated by Guruswami, Hastad and Sudan [SIAM J. Comp. 2002] and Dinur and Kol [CCC'13]. The covering number of a CSP instance $\Phi$ is the smallest number of assignments to the variables of $\Phi$, such that each constraint of $\Phi$ is satisfied by at least one of the assignments. We show the following results: 1. Assuming a covering variant of the Unique Games Conjecture, introduced by Dinur and Kol, we show that for every non-odd predicate $P$ over any constant-size alphabet and every integer $K$, it is NP-hard to approximate the covering number within a factor of $K$. This yields a complete characterization of CSPs over constant-size alphabets that are hard to cover. 2. For a large class of predicates that are contained in the 2k-LIN predicate, we show that it is quasi-NP-hard to distinguish between instances with covering number at most 2 and those with covering number at least $\Omega(\log\log n)$. This generalizes and improves the 4-LIN covering hardness result of Dinur and Kol.