elementary set theory - How to Read Notation for General Intersection and Union - Mathematics Stack Exchange
[University] Set Theory - Union and Intersection
Math Proofs: Definitions of Intersection and Union
How do you remember set theory basic symbols?
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Well, first you have it wrong unless you say that $I=\{1,\dots,n\}$. Then the statement simplifies:
The union of $A_i$ for $i\in I$ is the set of all such $x$ that there exists $i\in I$ with $x\in A_i$.
The intersection of $A_i$ for $i\in I$ is the set of all such $x$ that for all $i\in I$ we have $x\in A_i$.
You're almost right, just all the sets is a very unfortunate (and actually wrong) thing to say here, $I$ is a single set (of indices).
A better version is
$$\bigcup_{i=1}^n A_i=\{x|\text{ for some }j\in \{1,\ldots n\},x\in A_j\}$$
$$\bigcap_{i=1}^n A_i=\{x|\text{ for all }j\in \{1,\ldots n\},x\in A_j\}$$
and from there the "plain english" version follow naturally. The union includes anything in any of the sets. The intersection contains only those things in every set.
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Having trouble understanding my lecture notes and the specific notation they use. For context, this is the first time the union and intersection symbols appear. I'm pretty sure they big U and big upside down U mean 'union' and 'intersection' respectively. But I don't understand (We define U A (sub lambda) to be the set whose elements that belong to at least one of the A (sub lambda)). I find the wording to be incredibly confusing, and if anyone could please explain it in simpler terms. Any help is greatly appreciated.