- #1
gotjrgkr
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Homework Statement
Hi!
I'd like to ask you a definition of a morphism which is used to define the category.
I refer to a book "introduction to topological manifolds" by author Lee.
In his book, the concept of the category is introduced in p. 170.
I'll write it below.
A category C consists of the following:
1. a class (not necessarily a set)of objects;
2. for each pair of objects X,Y, a set Hom[itex]_{C}[/itex](X,Y) of morphisms; and
3. for each triple X,Y,Z of objects a function called composition :
Hom[itex]_{C}[/itex](X,Y)xHom[itex]_{C}[/itex](Y,Z)[itex]\rightarrow[/itex]Hom[itex]_{C}[/itex](X,Z), written (f,g)[itex]\rightarrow[/itex]g[itex]\circ[/itex]f;
such that the following axioms are satisfied;
(i) Composition is associative: (f[itex]\circ[/itex]g)[itex]\circ[/itex]h=f[itex]\circ[/itex](g[itex]\circ[/itex]h).
(ii) For each object X there exists an identity morphism Id[itex]_{X}[/itex][itex]\in[/itex]
Hom[itex]_{C}[/itex](X,X) such that for any morphism f[itex]\in[/itex]Hom[itex]_{C}[/itex](X,Y) we have Id[itex]_{Y}[/itex][itex]\circ[/itex]f=f=f[itex]\circ[/itex]Id[itex]_{X}[/itex].
In the above definition, I don't know what morphisms actually are...
I expect that it should be a function which is defined between classes. As you know, like functions between two sets in ZFC set theory, a function also can be defined between two classes if we admit NBG set theory. Do you think I am right? If not, what are exactly morphisms in the above definition??
Thanks a lot for reading my questions!