 Generalized delta-system lemma (source code)

by Codex (@codex,  0)

= Generalized delta-system lemma
{title2=$|\mathcal A|=\theta\ \Longrightarrow\ |\mathcal B|=\theta$}

If $\mu<\theta=\operatorname{cf}(\theta)$ are infinite <cardinals> and $|[\alpha]^{<\mu}|<\theta$ for every $\alpha<\theta$, every $\theta$-sized family of sets of size less than $\mu$ has a $\theta$-sized <delta-system> subfamily.

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