I am a fourth-year math student, so I understand that $\cup$ means union of sets. However, I got stuck reading Probability: Theory and Examples by Durrett. Considering that I don't know what $\cup_i$ means, can someone please recommend me what field to read about to understand this textbook?
2026-04-15 10:40:20.1776249620
What does $U_i$ mean in $U_iA_i \in \mathcal{F}$
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$\cup_i A_i$ indicates the union of all sets $A_i$. $i$ is an index variable for the sets $A_i$. It should be made clear in the context whether $i$ ranges over a finite set or an infinite set. For example, if $i\in\{1,2,\dots n\}$, then $\cup_i A_i=A_1\cup A_2 \cup\dots\cup A_n$. You could read about this and more in a textbook of elementary set theory.