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Sunflower (mathematics)
In mathematics, a sunflower or Δ system is a collection of sets whose pairwise intersection is constant, and called the kernel.
The Δ lemma, sunflower lemma, and sunflower conjecture give various conditions that imply the existence of a large sunflower in a given collection of sets.
The original term for this concept was "Δ-system". More recently the term "sunflower", possibly introduced by , has been gradually replacing it.
== Formal definition ==
Suppose ''U'' is a universe set and ''W'' is a collection of subsets of ''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, ''W'' is a sunflower if the pairwise intersection of each set in ''W'' is constant.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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