Searching for Frequent Colors in Rectangles
  We study a new variant of colored orthogonal range searching problem: given a query rectangle $Q$ all colors $c$, such that at least a fraction $\tau$ of all points in $Q$ are of color $c$, must be reported. We describe several data structures for that problem that use pseudo-linear space and answer queries in poly-logarithmic time. 
Marek Karpinski, Yakov Nekrich