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The modular decomposition of a five-cycle with each vertex blown up into an edgeless graph
Example
Let be obtained from the five-cycle by substituting an edgeless graph , with , for each of its five vertices. Then the five blown-up parts are exactly the maximal proper modules of , and the quotient graph is .
Facts & Assumptions
Given: An integer , the five-cycle , and the graph obtained by replacing each vertex of by a copy of .
A vertex set is a module when every vertex outside is complete or anticomplete to it (Modules of a graph, and the trivial modules).
The five-cycle is prime (The five-cycle is prime).
In a connected and anticonnected finite simple graph with at least two vertices, every modular partition with at least two parts and prime quotient consists of the maximal proper modules (In a connected and anticonnected graph, a modular partition with at least two parts whose quotient is prime consists of the maximal proper modules).
Verification
Each blown-up copy of is a module of : every vertex outside one part lies in another part, and that whole outside part is either complete or anticomplete to the chosen part according to the corresponding adjacency in the five-cycle. Hence every outside vertex is complete or anticomplete to the chosen part, so [L1] applies.
The graph is connected: paths between blown-up parts lift from paths in , and two vertices in one edgeless part have a common neighbour in either adjacent part. Its complement is connected by the same argument, because complementation turns the quotient into and each blown-up part into a clique. Thus is connected and anticonnected.
The quotient obtained by collapsing those five modules is the original five-cycle, so [L2] shows that this quotient is prime.
The five blown-up parts form a modular partition with at least two parts and prime quotient by steps 1.1 and 2.1. Hence [L3] and step 1.2 show that they are exactly the maximal proper modules, and the quotient graph is .
Depends on
- Modular partitions and the quotient graph they define
- In a connected and anticonnected graph, a modular partition with at least two parts whose quotient is prime consists of the maximal proper modules
- Substituting one graph for a vertex of another
- The five-cycle is prime
- Empty and complete graphs, complete bipartite graphs, and the convention that $P_n$ and $C_n$ have $n$ vertices
- Modules of a graph, and the trivial modules
Used by
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