# Reference for Lindelöf number of topological space

I try to study the Lindelöf number of topological space. Are there some classical books or papers or useful links on this? Thanks!

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Of course all of those references are at least $25$ years old; a search on Lindelöf degree and Lindelöf number will turn up a great deal of more recent material on this and related topics. But they should still be a useful starting point.