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Let G be a finite group. If A and B are two conjugacy classes in G, then AB is a union of conjugacy classes in G and h(AB) denotes the number of distinct conjugacy classes of G contained in AB. If c and y are two complex irreducible characters of G, then cy is a character of G and again we let h(cy) be the number of irreducible characters of G appearing as constituents of cy. In this paper our aim is to study the product of conjugacy classes in a finite group and obtain an upper bound for h in general. Then we study similar results related to the product of two irreducible characters.

Let R be a ring with unity. The nilpotent graph of R, denoted by GN(R), is a graph with vertex set ZN(R)* = {0 \neq x \in R \mid xy \in N(R) for some 0 \neq y \in R}; and two distinct vertices x and y are adjacent if and only if xy \in N(R), where N(R) is the set of all nilpotent elements of R. Recently, it has been proved that if R is a left Artinian ring, then diam(GN(R)) \leq 3. In this paper, we present a new proof for the above result, where R is a finite ring. We study the diameter and the girth of matrix algebras. We prove that if F is a field and n \geq 3, then diam(GN(Mn(F))) = 2. Also, we determine diam (GN (M2(F))) and classify all finite rings whose nilpotent graphs have diameter at most 3. Finally, we determine the girth of the nilpotent graph of matrix algebras.