Rainbow coloring hardness via low sensitivity polymorphisms

A k-uniform hypergraph is said to be r-rainbow colorable if there is an r-coloring of its vertices such that every hyperedge intersects all r color classes. Given as input such a hypergraph, finding a r-rainbow coloring of it is NP-hard for all k ≥ 3 and r ≥ 2. Therefore, one settles for finding a rainbow coloring with fewer colors (which is an easier task). When r = k (the maximum possible value), i.e., the hypergraph is k-partite, one can efficiently 2-rainbow color the hypergraph, i.e., 2-color its vertices so that there are no monochromatic edges. In this work we consider the next smaller value of r = k− 1, and prove that in this case it is NP-hard to rainbow color the hypergraph with q := d k−2 2 e colors. In particular, for k ≤ 6, it is NP-hard to 2-color (k − 1)-rainbow colorable k-uniform hypergraphs. Our proof follows the algebraic approach to promise constraint satisfaction problems. It proceeds by characterizing the polymorphisms associated with the approximate rainbow coloring problem, which are rainbow colorings of some product hypergraphs on vertex set [r]. We prove that any such polymorphism f : [r] → [q] must be C-fixing, i.e., there is a small subset S of C coordinates and a setting a ∈ [q] such that fixing x|S = a determines the value of f(x). The key step in our proof is bounding the sensitivity of certain rainbow colorings, thereby arguing that they must be juntas. Armed with the C-fixing characterization, our NP-hardness is obtained via a reduction from