In 
mathematics, 
general topology or 
point set topology is that branch of 
topology which studies elementary properties of 
topological spaces and structures defined on them.
Other main branches of topology are 
algebraic topology and 
differential topology.
It's in general topology that basic notions, such as:
are defined and theorems about them are proved.
Other more advanced notions also appear, but are usually related directly to these fundamental concepts, without reference to other branches of mathematics.
 Standard references 
Some standard books on general topology include:
 
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