A connection between covers of the integers and unit fractions
  For integers a and n>0, let a(n) denote the residue class {x\in Z: x=a (mod n)}. Let A be a collection {a_s(n_s)}_{s=1}^k of finitely many residue classes such that A covers all the integers at least m times but {a_s(n_s)}_{s=1}^{k-1} does not. We show that if n_k is a period of the covering function w_A(x)=|{1\le s\le k: x\in a_s(n_s)}| then for any r=0,...,n_k-1 there are at least m integers in the form $\sum_{s\in I}1/n_s-r/n_k$ with I contained in {1,...,k-1}. 
Zhi-Wei Sun