Packing, Hitting, and Coloring Squares

Given a family of squares in the plane, their packing problem asks for the maximum number, $\nu$, of pairwise disjoint squares among them, while their hitting problem asks for the minimum number, $\tau$, of points hitting all of them, $\tau \ge \nu$. Both problems are NP-hard even if all the rectangles are unit squares and their sides are parallel to the axes. The main results of this work are providing the first bounds for the $\tau / \nu$ ratio on not necessarily axis-parallel squares. We establish an upper bound of $6$ for unit squares and $10$ for squares of varying sizes. The worst ratios we can provide with examples are $3$ and $4$, respectively. For comparison, in the axis-parallel case, the supremum of the considered ratio is in the interval $[\frac{3}{2},2]$ for unit squares and $[\frac{3}{2},4]$ for arbitrary squares. The new bounds necessitate a mixture of novel and classical techniques of possibly extendable use. Furthermore, we study rectangles with a bounded "aspect ratio", where the aspect ratio of a rectangle is the larger side of a rectangle divided by its smaller side. We improve on the well-known best $\tau/\nu$ bound, which is quadratic in terms of the aspect ratio. We reduce it from quadratic to linear for rectangles, even if they are not axis-parallel, and from linear to logarithmic, for axis-parallel rectangles. Finally, we prove similar bounds for the chromatic numbers of squares and rectangles with a bounded aspect ratio.