The formal definition section is good: Suppose W is a set system over U, that is, a collection of subsets of a set U. The collection W is a sunflower (or Δ-sys

The formal definition section is good: Suppose W is a set system over U, that is, a collection of subsets of a set U. The collection W is a sunflower (or Δ-system) if there is a subset S of U such that for each distinct A and B in W, we have A ∩ B = S. In other words, a set system or collection of sets W is a sunflower if the pairwise intersection of each set in W is identical. Note that this intersection, S, may be empty; a collection of pairwise disjoint subsets is also a sunflower. Similarly, a collection of sets each containing the same elements is also trivially a sunflower.