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October 25, 2017 / porton

Two kinds of generalization

I noticed that there are two different things in mathematics both referred as “generalization”.

The first is like replacing real numbers with complex numbers, that is replacing a set in consideration with its superset.

The second is like replacing a metric space with its topology, that is abstracting away some properties.

Why are both called with the same word “generalization”? What is common in these two? Please comment.


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  1. Jon Awbrey / Oct 25 2017 19:00

    One path to generalization passes from a smaller space to a larger space.
    Another path to generalization passes from more axioms to fewer axioms.

    They sometimes converge, sometimes not.

    I tried making this point on MathOverFlow once but it was not well received.

    On a related note, C.S. Peirce recognized two types of abstraction —

    Prescisive Abstraction

    Hypostatic Abstraction


  2. GV / Oct 26 2017 00:16

    Isn’t the “set” of topological spaces a “superset” of the “set” of metric spaces?


    • porton / Oct 26 2017 00:30

      It isn’t. Multiple metric spaces correspond to a single topology not vice versa.


  3. Anonymous Coward / Nov 2 2017 12:11

    I think mathematicians outside category theory often treat structures as if they were properties. For example, they treat metric spaces as just “metrizable topological spaces”. In that regard, then forgetting a structure is kind of like generalizing to the superset which may not have that selected property.
    It seems to me that category theorists have made this quite precise here: .


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