But there are differences, and they cannot be combined at will. A small u that can only represent a combination of finite items cannot represent that situation. So in mathematics, a big U is often used to mean "tired connection". The cross is similar.
In mathematics, if there is an operation symbol a, then a1aa2a3a. . . An is generally used in mathematics.
The big A Ai i is represented by the symbol from 1 to n, which can be easily extended to countless An "A" s together, and can also be easily extended to more general arbitrary term "A" if the definition is meaningful.
Above, you can use "intersection" or "union" instead of "a". Only addition and multiplication have special Greek letters for accumulation and multiplication, and other operations are generally completed by amplifying their own symbols.
It can be used in probability theory and mathematical statistics. But this is very important for pure mathematics, because many problems need to discuss the difference between infinite dimension and arbitrary finite dimension. You'll need this.