The relation between equiangular sets of lines in the real space and distance-regular double covers of the complete graph is well known and studied since the work of Seidel and others in the 70's. The main topic of this paper is to continue the study on how complex equiangular lines relate to distance-regular covers of the complete graph with larger index. Given a set of equiangular lines meeting the relative (or Welch) bound, we show that if the entries of the corresponding Gram matrix are prime roots of unity, then these lines can be used to construct an antipodal distance-regular graph of diameter three. We also study in detail how the absolute (or Gerzon) bound for a set of equiangular lines can be used to derive bounds of the parameters of abelian distanceregular covers of the complete graph.

Let $G = (V,E)$ be a graph, and let $f : V \rightarrow 2^{\Bbb Z}$ be a function assigning to each $v \in V$ a set of integers in $\{0,1,2,\dots,d(v)\}$, where $d(v)$ denotes the degree of $v$ in $G$. Lovász defines an $f$-factor of $G$ to be a spanning subgraph $H$ of $G$ in which $d_{H}(v) \in f(v)$ for all $v \in V$. Using the combinatorial nullstellensatz of Alon, we prove that if $|f(v)| > \lceil {1\over 2}d(v) \rceil$ for all $v \in V$, then $G$ has an $f$-factor. This result is best possible and verifies a conjecture of Addario-Berry, Dalal, Reed and Thomason.