Published online by Cambridge University Press: 30 May 2022
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
$F$. 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 Häggkvist. If
$G$ is a spanning subgraph of the blow-up of
$C_k$ with parts of size
$n$ and
$\delta ^*(G) \ge \left(1 + \frac 1k\right)\frac n2 + 1$, then
$G$ contains
$n$ vertex disjoint copies of
$C_k$ such that each
$C_k$ intersects each of the
$k$ parts exactly once. A similar conjecture was also made by Fischer and the case
$k=3$ was proved for large
$n$ by Magyar and Martin.
In this paper, we prove the conjecture of Häggkvist asymptotically. We also pose a conjecture which generalises this result by allowing the minimum degree conditions in each bipartite subgraph induced by pairs of parts of $G$ to vary. We support this new conjecture by proving the triangle case. This result generalises Johannson’s result asymptotically.
Research supported in part by NSF Grant DMS 1800761.