Degenerations and order of graphs realized by finite abelian groups

Abstract

Let G_1 and G_2 be two groups. If a group homomorphism \varphi : G_1 \longrightarrow G_2 maps a \in G_1 into b \in G_2 such that \varphi(a) = b, then we say a degenerates to b and if every element of G_1 degenerates to elements in G_2, then we say G_1 degenerates to G_2. In this paper, we study degeneration in graphs and show that degeneration in groups is a particular case of degeneration in graphs. We exhibit some interesting properties of degeneration in graphs. We use this concept to present a pictorial representation of graphs realized by finite abelian groups. We discus some partial orders on the set T_p_1 \dots T_p_n of all graphs realized by finite abelian p_r-groups, where each p_r, 1 \leq r \leq n, is a prime number. We show that each finite abelian p_r-group of rank n can be identified with saturated chains of Young diagrams in the poset T_p_1 \dots T_p_n. We present a combinatorial formula which represents the degree of a projective representation of a symmetric group. This formula determines the number of different saturated chains in T_p_1 \cdots T_p_n and the number of finite abelian groups of different orders.