Enclosings of Decompositions of Complete Multigraphs in 2-Factorizations

Let $k$, $λ$ and $μ$ be positive integers. A decomposition of a multigraph $λG$ into edge-disjoint subgraphs $G_1, \ldots , G_k$ is said to be \emph{enclosed} by a decomposition of a multigraph $μH$ into edge-disjoint subgraphs $H_1, \ldots , H_k$ if $μ> λ$ and $G_i$ is a subgraph of $H_i$, $1 \leq i \leq k$. In this paper we initiate the study of when a decomposition can be enclosed by a decomposition that consists of spanning subgraphs. A decomposition of a graph is a 2-factorization if each subgraph is 2-regular and is Hamiltonian if each subgraph is a Hamiltonian cycle. Let $n$ and $m$ be positive integers. We give necessary and sufficient conditions for enclosing a decomposition of $λK_n$ in a $2$-factorization of $μK_{n+m}$ whenever $μ>λ$ and $m \geq n-2$. We also give necessary and sufficient conditions for enclosing a decomposition of $λK_n$ in a Hamiltonian decomposition of $μK_{n+m}$ whenever $μ> λ$ and $m \geq n-1$, or $μ> λ$, $n=3$ and $m=1$, or $μ= 2$, $λ=1$ and $m=n-2$.

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