Hello! I just started reading an introductory book about topology and I got a bit confused from the definition. One of the condition for a topological space is that if ##\tau## is a collection of subsets of X, we have {##U_\alpha | \alpha \in I##} implies ##\cup_{\alpha \in I} U_\alpha \in \tau ##. I assume this means that for any 2 sets in ##\tau## their union is also in ##\tau##. But I really don't understand the notation. What does {##U_\alpha | \alpha \in I##} mean? And how is it related to ##\tau##? And what is I? There is nothing before this, to define "I" and I found this definition in different books, so I assume i am missing something here. Thank you!(adsbygoogle = window.adsbygoogle || []).push({});

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# I Definition of topology

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