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Signed-off-by: zeramorphic <[email protected]>
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zeramorphic committed Nov 20, 2023
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2 changes: 1 addition & 1 deletion iii/cat/03_adjunctions.tex
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Expand Up @@ -15,7 +15,7 @@ \subsection{Definition and examples}
\[ \mathbf{Top}(DX, Y) \leftrightarrow \mathbf{Set}(X, UY) \]
It also has a right adjoint \( I : \mathbf{Set} \to \mathbf{Top} \) which equips each set with its indiscrete topology.
\[ \mathbf{Set}(UX, Y) \leftrightarrow \mathbf{Top}(X, IY) \]
\item Consider the functor \( \ob : \mathbf{Cat} \to \mathbf{Set} \) which maps each category to each set of objects.
\item Consider the functor \( \ob : \mathbf{Cat} \to \mathbf{Set} \) which maps each category to its set of objects.
It has a left adjoint \( D \) which turns each set \( X \) into a discrete category in which the objects are elements of \( X \), and the only morphisms are identities.
It also has a right adjoint \( I \) which turns each set \( X \) into an indiscrete category in which the objects are elements of \( X \), and there is exactly one morphism between any two elements of \( X \).
In addition, \( D : \mathbf{Set} \to \mathbf{Cat} \) has a left adjoint \( \pi_0 : \mathbf{Cat} \to \mathbf{Set} \), where \( \pi_0 \mathcal C \) is the set of connected components of \( \ob \mathcal C \) under the graph induced by its morphisms.
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2 changes: 1 addition & 1 deletion iii/cat/06_monoidal_and_enriched_categories.tex
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Expand Up @@ -59,7 +59,7 @@ \subsection{Monoidal categories}
\arrow["{1_A \otimes \rho_B}", from=1-2, to=2-2]
\arrow["{\rho_{A \otimes B}}"', from=1-1, to=2-2]
\end{tikzcd}\]
Note that in the category of abelian groups with the usual tensor product, the obvious choice for \( \alpha \) is the map sending \( (a \otimes b) \otimes c \) to \( a \otimes (b \otimes c) \).
Note that in the category of abelian groups with the usual tensor product, the obvious choice for \( \alpha_{A,B,C} \) is the map sending \( (a \otimes b) \otimes c \) to \( a \otimes (b \otimes c) \).
However, there is also a natural isomorphism sending \( (a \otimes b) \otimes c \) to \( -a \otimes (b \otimes c) \).
But this choice does not satisfy the pentagon equation, as a pentagon has an odd number of sides.

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