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If a connected graph is 2''k''-regular and has an even number of edges it may also be ''k''-factored, by choosing each of the two factors to be an alternating subset of the edges of an [[Euler tour]].<ref>{{harvtxt|Petersen|1891}}, §6, p. 198.</ref> This applies only to connected graphs; disconnected counterexamples include disjoint unions of odd cycles, or of copies of ''K''<sub>2''k''+1</sub>.
The [[Oberwolfach problem]] concerns the existence of 2-factorizations of [[complete graph]]s into isomorphic subgraphs. It asks for which subgraphs this is possible. This is known when the subgraph is connected (in which case it is a [[Hamiltonian cycle]] and this special case is the problem of [[Hamiltonian decomposition]]) but the general case remains unsolved.
==Notes==
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