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So if $\mathcal{C}$ is locally small, we have the Yoneda embedding $Y:\mathcal{C} \rightarrow [\mathcal{C}^{op},Sets ]$. This preserves all limits in $\mathcal{C}$, and a comment here: An application of Yoneda Lemma says that showing this is just a matter of reformulating the definition of a limit. How is this?

I thought about how I would show that $Y$ preserves limits, and all I could think of was this: Suppose $L$ is the limit in $\mathcal{C}$ of some diagram $D$. Take some cone over $YD$ in $[\mathcal{C}^{op},Sets]$ with base $P$ and write $P$ as a colimit of representable presheaves. (I think) you can then use the Yoneda embedding to show that $L$ is a cocone over the elements of $P$, and the unique map $P \rightarrow YL$ from the universal property of $P$ will also work to show that $YL$ is indeed the limit.

Is this proof correct, and what is the easy proof that I've missed?

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up vote 7 down vote accepted

If $\{L \to D_i\}$ is a limit diagram in $C$, then for every $X \in C$ we have that $\{\hom(X,L) \to \hom(X,D_i)\}_i$ is a limit diagram in $\mathsf{Set}$, which means precisely that $\{\hom(-,L) \to \hom(-,D_i)\}_i$ is a limit diagram in $[C^{op},\mathsf{Set}]$.

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Thanks. The fact that I didn't see that means I need to go understand the Yoneda lemma better, I suppose... –  Zach L. Jun 19 '13 at 14:38
    
You only need the definition of $Y$, not anything else. –  Martin Brandenburg Jun 19 '13 at 17:53
    
For a categorical novice, it might be worthwhile to explicitly state that the last step requires not just that $\{\hom(X,L) \to \hom(X,D_i)\}_i$ is a limit diagram, but that for each $X$, but also that the morphisms in these diagrams are natural in $X$. –  Omar Antolín-Camarena Jun 19 '13 at 18:39
    
Yes this is the existence of $Y$. –  Martin Brandenburg Jun 19 '13 at 19:42
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