irp.org/file/10-1200304x118.png" /> of length. Obviously,. Then
Now, let and let us assume that for all even graphs G such that the strict inequalities hold.
Let G be an even graph such that. We will show for all.
Let such that. Consider the graphs and. is even and. Obviously, and. But also must hold, otherwise would not be a minimizer of L on.
Hence,. Applying the induction assumption to, we then get:
Here is a minimizer of L on. From this we finally conclude
By, we denote the family of all cycle packings of G. We get
Theorem 1. Let G be even,. Every cycle packing that minimizes L on is maximum, i.e..
Proof. Let be a minimizer of L on. We can assume that We will show that.
For this, consider the non-even graph obtained by adding a component and an additional edge to G. Obviously, is even and. For the packing with it holds and. We will show .
For an arbitrary packing, the remainder is the dis- joint union of and some even graph H that contains at least one cycle of length. Let.
If the component is not contained in H, then is one of the cycles, i.e.. In this case C is a cycle in G.
Consider the packing Then . We get
We conclude, that there must be a minimizer of L on, such that. Obviously, , i.e. with.
Applying Lemma 1 to the even graph, we get for:
where the last inequality is strict if.
Now, let be a maximum cycle packing that mini-
mizes L on, i.e..
Then with minimizes L on
. Obviously,. We get
where the inequality is strict, if.
It follows. But was a minimizer on, i.e.. Therefore, and.
A maximum cycle packing of G is said to be max-min if it minimizes L on. The quantity is the max-min cycle value of G. The determination of a max-min cycle packing will be called the max-min cycles packing problem (mmcp-problem) of G. Clearly, max-min cycle packings, in general, are not unique.
The following theorem relates the determination of to the determination of the max-min cycle values for even subgraphs.
Theorem 2. Let G be even. Then
Proof. Let be an even subgraph of G and be max-min.
A packing then induces a
packing. We then get
Now, let be max-min and let and be induced by and, respectively.
The packings and must also be max-min. We get
The proof of Theorem 2 immediately induces
1), for every even subgraph H of G.
3. A Shortest Path Approach for the MMCP-Problem
Theorem 2 gives reason to treat the mmcp-problem as a shortest path problem within a suitable weighted acyclic network
3.1. The MMCP-Network
Let the edges in E be labelled, i.e.. In a canonical way, a subgraph is determined by its incidence vector
1For we will use “nodes”, in G we use “vertices”.
We will identify the set X of nodes1 in with the set of even subgraphs of G. Each node corresponds to some specific even subgraph H of G (we will write). Nodes in are also assigned to stages, i.e. .
For the construction of, the nodes and edges are defined inductively:
・ The unique node in corresponds to the subgraph of G with empty edge set. Assume that the set of nodes is given. Then a node belongs to if there is such that
We call to be a successor of. The set of all successors of is called an expansion of, and a specific successor is generated by expanding.
・ An edge in U corresponds to some specific cycle in G. Edges exist only between nodes at consecutive stages. An edge if and only if for the cor- responding subgraphs is a successor of.
・ As edge weights we set.
Clearly, is acyclic and the number of stages in cannot exceed. An even subgraph is reachable in if there is a path with starting node and end node H. In a canonical way, any path induces a cycle packing. The set describes the cycles used in the successive expansions of the corresponding nodes.
Obviously, G is reachable in, but not all even subgraphs of G have this property. Hence, not every cycle packing is induced by some paths. We get
Lemma 2. Let be reachable in and be a cycle-packing of H of cardinality s. Then there is a path in that induces.
Proof. Let be a cycle packing of H of cardinality. Without loss of generality, we can assume that the cycles in are ordered such that. Let and be the even subgraph of H induced by. Since the cycles are mutually edge-disjoint, the number coincides with. Therefore, is generated by an expansion of, i.e. there is a path that induces. Moreover, if and are two different cycle-packings of H, then the corresponding paths in must be different.
Together with Theorem 1, we conclude for the special case:
Proposition 3. is a max-min cycle packing of G if and only if corresponds to a shortest path in.
In order to reduce the number of graphs that have to be expanded within the algorithmic procedure, those subgraphs H in have to be identified that cannot be contained in. Such an identification, preferably, should be done as early as possible in the calculations. The following proposition gives conditions for such a situation. They can be checked during the shortest path procedure. If such a condition is satisfied, H must never be expanded.
Proposition 4. Let be a shortest path in and let be reachable. If
Proof. Since, holds. Assume that. Then there is a cycle packing induced by. The packing, therefore, must be max-min. Hence,. This leads to the contradiction.
3.2. An A*-Shortest Path Algorithm
For an even subgraph, let denote the length of the shortest cycle in H, then,
is a lower bound for the max-min cycle value. Moreover, the function is a monotonous node potential on in the following sense
Lemma 3. For, let be two even subgraphs of G adjacent in such that. Then
Proof. Let and be the lengths of the shortest cycles in and, respectively. Then
The last inequality is true, since. Hence,
In  , it is described how information of a monotonous node potential could be incorporated into a searching strategy for the shortest path procedure. Such an - search algorithm constructs successively and expands only such nodes that are candidates for a shortest path.
The procedure essentially manages two sets of nodes in and updates several quantities:
・ X: even subgraphs of G that are candidates to determine.2
・ L: subgraphs that are already expanded. For, a shortest path is already constructed.
・ : index of the stage in at which H is considered. If terminates, is the cardinality of a maximum cycle packing.
・ : (currently) the shortest length of a path. If, then. If stops,.
・ : monotonous node potential.
The scheme of such an -search is outlined as follows.
The determination of in step 1 requires the determination of the girth of. This can efficiently be done by the shortest paths procedures. For the expansion of H in step 2, the value and the set C of all cycles in that contain must be generated. This makes it necessary to identify all simple paths between u and v in the graph. Typically, this subproblem is attacked by using DFS procedures. In general, it is NP-hard (  ).
Step 3 incorporates the stopping rule () and the elimination of super- fluous nodes (and sub-paths) according to Prop. 4.
Coming from step 2, terminates in step 3 if G is expanded from some H for the first time. Since it is possible that the graph G may appear in at different stages, it must be guaranteed that doesn’t stop at a “wrong” node that corresponds to G.
Proposition 5. If stops, then is a max-min cycle packing of G.
Proof. Assume, terminates and. In this case cor- responds to a node at stage. Since the corresponding cycle packing is not maximum,. Let be the path in induced by and let be the predecessor of on that path.
The only node in at stage corresponds to G (). For the shortest path, Theorem 1 gives.
Let be the last common node of the paths and. Since is expanded at some iteration of, there must be a subgraph on the subpath of that belongs to X when terminates. Since this node is never selected in step 1 until stops (otherwise it would have been deleted from X in step 2), the inequality must hold. But then
is a contradiction. Hence, terminates at stage, i.e. is maximum.
We thank the Editor and the referees for their comments.
2We will write “H Î X”, indicating that the node that corresponds to H belongs to X.