diff --git a/Tennenbaum/Tennenbaum.tex b/Tennenbaum/Tennenbaum.tex index f7aca5c..4a70364 100644 --- a/Tennenbaum/Tennenbaum.tex +++ b/Tennenbaum/Tennenbaum.tex @@ -157,4 +157,4 @@ This contradicts the undecidability of $X$. \section{Acknowledgements} The structure of the proof is from Dr Thomas Forster's lecture notes on Computability and Logic from Part III of the Cambridge Maths Tripos, lectured in 2016. -\end{document} \ No newline at end of file +\end{document} diff --git a/YonedaWithoutTears/YonedaWithoutTears.tex b/YonedaWithoutTears/YonedaWithoutTears.tex index d9cb59c..3e2c446 100644 --- a/YonedaWithoutTears/YonedaWithoutTears.tex +++ b/YonedaWithoutTears/YonedaWithoutTears.tex @@ -55,7 +55,7 @@ \begin{definition} Let $F, G: \mathcal{C} \to \mathcal{D}$ be functors. A \emph{natural transformation} from $F$ to $G$ is a selection $\alpha$, parametrised over each $X \in \mathcal{C}$, of an arrow $\alpha_X$ in $\mathcal{D}$ from $FX \to GX$. - We require this selection to be ``natural'' in that whenever $f : X \to Y$ is an arrow in $\mathcal{C}$, we have $$FX \xrightarrow{Ff} FY \xrightarrow{\alpha_Y} GY = FX \xrightarrow{\alpha_X} GX \xrightarrow{Gf} GY$$ + We require this selection to be ``natural'' in that whenever $f : X \to Y$ is an arrow in $\mathcal{C}$, we have $$(FX \xrightarrow{Ff} FY \xrightarrow{\alpha_Y} GY) = (FX \xrightarrow{\alpha_X} GX \xrightarrow{Gf} GY)$$ \end{definition} \ @@ -74,7 +74,7 @@ \begin{thm}[The Yoneda lemma] Let $\mathcal{C}$ be a category, and let $G: \mathcal{C} \to \Set$ be a functor. Let $A$ be an object of $\mathcal{C}$. - Then $$\Nat [\homfrom{A} \to G ] \cong G A$$ + Then $$\Nat [\homfrom{A}, G] \cong G A$$ and moreover the bijection is natural in both $G$ and $A$. \end{thm} @@ -121,7 +121,7 @@ (Note that this is not quite what is usually meant by a model homomorphism, and I have invented the term ``fixed model homomorphism'' to describe it.) A \emph{fixed model homomorphism} $\alpha$ is a function from one model $F: \mathcal{C} \to \Set$ of the theory $\mathcal{C}$ to another model $G: \mathcal{C} \to \Set$, which assigns to each type $FA$ of the model $F$ the corresponding type $GA$ of the model $G$, in such a way that $\alpha$ respects the predicates $Ff: FA \to FB$ of the model: - $$FA \xrightarrow{Ff} FB \xrightarrow{\alpha_B} GB = FA \xrightarrow{\alpha_A} GA \xrightarrow{Gf} GB$$ + $$(FA \xrightarrow{Ff} FB \xrightarrow{\alpha_B} GB) = (FA \xrightarrow{\alpha_A} GA \xrightarrow{Gf}) GB$$ Notice that this is a model homomorphism which additionally ensures that $FA$ is always mapped to $GA$ (for any $A$), so (for example) it won't collapse all the objects $FA$ into a single object in $G$'s image unless $G$ is the trivial model. @@ -130,7 +130,7 @@ Notice that this is a model homomorphism which additionally ensures that $FA$ is So the second key insight is that a natural transformation between functors $F: \mathcal{C} \to \Set$ and $G$ is just a fixed homomorphism between the $\Set$-models $F$ and $G$ of the theory $\mathcal{C}$. \section{Free models} - Throughout mathematics, there is the notion of a free object: an object which somehow has the least possible structure while still obeying all the rules it has to obey. + Throughout mathematics, there is the notion of a free object: an object which somehow has the least possible restriction while still obeying all the rules it has to obey. Can we find a free model of the theory represented by the category $\mathcal{C}$? Imagine $\mathcal{C}$ has two objects. Then any free model worth its name must have at least two types - otherwise we've definitely lost information in the model. @@ -253,4 +253,4 @@ I'm afraid I don't know of a good way to think about naturality other than just This entire document is derived from an answer by Sridhar Ramesh on a Math Overflow answer at \url{https://mathoverflow.net/a/15143}. -\end{document} \ No newline at end of file +\end{document}