 The collection formula_5 is a "sunflower" (or "formula_1-system") if there is a subset formula_9 of formula_4 such that for each distinct formula_11 and formula_12 in formula_5, we have formula_14.In other words, formula_5 is a sunflower if the pairwise intersection of each set in formula_5 is constant.Note that this intersection, formula_9, may be empty; a collection of disjoint subsets is also a sunflower.