In this question, any graph is a finite undirected graph which may have self-loops and multiple edges. Define the folowing terms and symbols:
For any graph , let and denote the number of vertices and edges in , respectively.
For any graph and any vertex in , denotes the degree of in .
For any graph and any walk on , the length of is the total number of edges that passes.
For a graph and a closed walk on , is a 1-circuit (or an Euler circuit) if passes each edge of exactly once, and is a 2-circuit if passes each edge of once or twice.
A subgraph of a graph is a parity subgraph if contains all vertices of and for any vertex in , and have the same odd-even parity. A parity subgraph of is a minimum parity subgraph if the number of edges in is minimum among all parity subgraphs of .
Answer the folowing questions. If necessary, the following facts (A) and (B) can be applied without proof.
(A) A connected graph has a 1-circuit if and only if every vertex of is of even degree.
(B) For any natural number , any tree of vertices has edges.
(1) Let be any connected graph.
(a) Prove that for any parity subgraph of , has a 2-circuit such that .
(b) Prove that for any 2-circuit in , has a parity subgraph such that .
(2) Let be any connected graph and be any minimum parity subgraph of .
(a) Prove that contains no cycle.
(b) Let denote the length of a shortest 2-circuit in . Prove that consists of connected components.
(a) From a parity subgraph to a 2-circuit and its length
Let be a parity subgraph. Form a multigraph by adding one parallel copy of every edge of to .
Then for each vertex ,
so every vertex of has even degree. By (A), has an Euler circuit .
Read back in by forgetting which of two parallel copies it used.
Every edge of is used once; every edge of is used twice.
Thus we obtain a 2-circuit in with
(b) From a 2-circuit to a parity subgraph and its size
Let be any 2-circuit of . Define to be the subgraph
consisting of the edges that uses twice.
Then
To see is a parity subgraph, fix a vertex . In the closed walk , each visit to uses two incident edge-ends (enter/leave), and a loop at contributes two ends; hence the total number of used edge-ends at is even. Modulo , this implies the number of incident edges used exactly once at is even, i.e.
If contained a cycle (a loop counts as a 1-cycle), removing all edges of would decrease the degree of each vertex on by , preserving parity at every vertex. The resulting subgraph would be a parity subgraph with fewer edges, contradicting the minimality of . Hence is acyclic.