Sunflower (mathematics)

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A mathematical sunflower can be pictured as a flower. The kernel of the sunflower is the brown part in the middle, and each set of the sunflower is the union of a petal and the kernel.

In mathematics, a sunflower or $\Delta$-system is a collection of sets whose pairwise intersection is constant, and called the kernel.

The $\Delta$-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 "$\Delta$-system". More recently the term "sunflower", possibly introduced by Deza & Frankl (1981), 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 $\Delta$-system) if there is a subset $S$ of $U$ such that for each distinct $A$ and $B$ in $W$, we have $A \cap B = S$. In other words, $W$ is a sunflower if the pairwise intersection of each set in $W$ is constant.

Δ-lemma

The $\Delta$-lemma states that every uncountable collection of finite sets contains an uncountable $\Delta$-system.

The $\Delta$-lemma is a combinatorial set-theoretic tool used in proofs to impose an upper bound on the size of a collection of pairwise incompatible elements in a forcing poset. It may for example be used as one of the ingredients in a proof showing that it is consistent with Zermelo-Fraenkel set theory that the continuum hypothesis does not hold. It was introduced by Shanin (1946).

Δ-lemma for ω₂

If $W$ is an $\omega_2$-sized collection of countable subsets of $\omega_2$, and if the continuum hypothesis holds, then there is an $\omega_2$-sized $\Delta$-subsystem. Let $\langle A_\alpha:\alpha<\omega_2\rangle$ enumerate $W$. For ${\rm cf}(\alpha)=\omega_1$, let $f(\alpha)={\rm sup}(A_\alpha\cap\alpha)$. By Fodor's lemma, fix $S$ stationary in $\omega_2$ such that $f$ is constantly equal to $\beta$ on $S$. Build $S'\subseteq S$ of cardinality $\omega_2$ such that whenever $i<j$ are in $S'$ then $A_i\subseteq j$. Using the continuum hypothesis, there are only $\omega_1$-many countable subsets of $\beta$, so by further thinning we may stabilize the kernel.

Sunflower lemma and conjecture

Erdős & Rado (1960, p. 86) proved the sunflower lemma, stating that if $a$ and $b$ are positive integers then a collection of $b!a^{b+1}$ sets of cardinality at most $b$ contains a sunflower with more than a sets. The sunflower conjecture is one of several variations of the conjecture of (Erdős & Rado 1960, p. 86) that the factor of $b!$ can be replaced by $C^b$ for some constant $C$.