 Corners theorem

For every $\delta >0$, all sufficiently large integers $n$ have the property that every $A\subseteq [n{]}^2$ with $|A|\ge \delta n^2$ contains a corner in an integer grid. The tripartite graph encoding of a grid turns the absence of a corner into a family of many edge-disjoint triangles but only quadratically many total triangles in a graph, contradicting the triangle removal lemma.

Corners theorem, often referred to in the context of graph theory and combinatorial geometry, generally deals with conditions on the arrangement of points or vertices in a specific geometric or combinatorial setting. The theorem states that given a finite set of points in the plane, one can find a subset of these points such that certain geometric or combinatorial properties hold, often involving the vertices (or corners) of a configuration.