Given a graph sequence {G n } n≥1 denote by T 3 (G n ) the number of monochromatic triangles in a uniformly random coloring of the vertices of G n with c ≥ 2 colors. In this paper we prove a central limit theorem (CLT) for T 3 (G n ) with explicit error rates, using a quantitative version of the martingale CLT. We then relate this error term to the well-known fourth-moment phenomenon, which, interestingly, holds only when the number of colors satisfies c ≥ 5. We also show that the convergence of the fourth moment is necessary to obtain a Gaussian limit for any c ≥ 2, which, together with the above result, implies that the fourth-moment condition characterizes the limiting normal distribution of T 3 (G n ), whenever c ≥ 5. Finally, to illustrate the promise of our approach, we include an alternative proof of the CLT for the number of monochromatic edges, which provides quantitative rates for the results obtained in [7].
KEYWORDSbirthday problem, fourth-moment theorem, graph coloring, martingale central limit theorem, rates of convergence.
INTRODUCTION AND MAIN RESULTS
Let) is an edge in G n and a ij (G n ) = 0 otherwise. In a uniformly random c-coloring of G n , the vertices of G n are colored with c ≥ 2 colors as follows: