Boundary Qoutinet Graphs and Graph Index
  In this paper, we define the quotinet graphs. In particular, we introduce the boundary quotient graphs, admissible boundary quotient graphs and subgraph boundary qoutient graphs. By the property of the quotient spaces, the boundary quotients need not be invariants on graphs. But, in certain cases, the admissible boundary quotient can be an invariant. The graph index is defined and observed. The concept is somewhat artificial, but we can construct the group-theory-like index on graphs, with respect to the subgraph qoutient graphs. 
Ilwoo Cho