Transversal $C_k$-factors in subgraphs of the balanced blow-up of $C_k$

Abstract

For a subgraph $G$ of the blow-up of a graph $F$, we let $\delta^*(G)$ be the smallest minimum degree over all of the bipartite subgraphs of $G$ induced by pairs of parts that correspond to edges of $F$. In [Triangle-factors in a balanced blown-up triangle. Discrete Mathematics, 2000], Johansson proved that if $G$ is a spanning subgraph of the blow-up of $C_3$ with parts of size $n$ and $\delta^*(G) \ge \frac{2}{3}n + \sqrt{n}$, then $G$ contains $n$ vertex-disjoint triangles, and presented the following conjecture of H\"aggkvist: If $G$ is a spanning subgraph of the blow-up of $C_k$ with parts of size $n$ and $\delta^*(G) \ge (1 + 1/k)n/2 + 1$, then $G$ contains $n$ vertex disjoint copies of $C_k$ such that each $C_k$ intersects each of the $k$ parts exactly once. The degree condition of this conjecture is tight when $k=3$ and cannot be strengthened by more than one when $k \ge 4$., A similar conjecture was also made by Fischer in [Variants of the Hajnal-Szemer\'edi Theorem. Journal of Graph Theory, 1999] and the triangle case was proved for large $n$ by Magyar and Martin in [Tripartite version of the Corr\'adi-Hajnal Theorem. Discrete Mathematics, 2002]. In this paper, we prove this Conjecture asymptotically. We also pose a conjecture which generalizes this result by allowing the minimum degree conditions on the nonempty bipartite subgraphs induced by pairs of parts to vary. Our second result supports this new conjecture by proving the triangle case. This result generalizes Johannson's result asymptotically.