Example 19.2.2. Next we give an example where the map fails to be injective. Let $B_ n = \mathbf{N}/\{ 1, 2, \ldots , n\}$, that is, the quotient set of $\mathbf{N}$ with the first $n$ elements collapsed to one element. There are natural maps $B_ n \to B_ m$ for $n \leq m$, so the $\{ B_ n\}$ form a system of sets over $\mathbf{N}$. It is easy to see that $\mathop{\mathrm{colim}}\nolimits B_ n = \{ *\}$: it is the one-point set. So it follows that $\mathop{\mathrm{Mor}}\nolimits (A, \mathop{\mathrm{colim}}\nolimits B_ n)$ is a one-element set for every set $A$. However, $\mathop{\mathrm{colim}}\nolimits \mathop{\mathrm{Mor}}\nolimits (A , B_ n)$ is not a one-element set. Consider the family of maps $A \to B_ n$ which are just the natural projections $\mathbf{N} \to \mathbf{N}/\{ 1, 2, \ldots , n\}$ and the family of maps $A \to B_ n$ which map the whole of $A$ to the class of $1$. These two families of maps are distinct at each step and thus are distinct in $\mathop{\mathrm{colim}}\nolimits \mathop{\mathrm{Mor}}\nolimits (A, B_ n)$, but they induce the same map $A \to \mathop{\mathrm{colim}}\nolimits B_ n$.

There are also:

• 2 comment(s) on Section 19.2: Baer's argument for modules

In your comment you can use Markdown and LaTeX style mathematics (enclose it like $\pi$). A preview option is available if you wish to see how it works out (just click on the eye in the toolbar).