- #1
NihilTico
- 32
- 2
Homework Statement
Coproducts exist in Grp. This starts on page 71. of his Algebra.
Homework Equations
[/B]
Allow me to present the proof in it's entirety, modified only where it's convenient or necessary for TeXing it. I've underlined areas where I have issues and bold bracketed off my snarky/expository asides.
Let [itex]\{G_i\}_{i\in I}[/itex] be a family of groups. We work in a category whose objects are families of group-homomorphisms [itex]\{g_i\colon G_i\to G\}_{i\in I}[/itex] and whose morphisms are the obvious ones. We must find a universal element in this category. For each index [itex]i[/itex], we let [itex]S_i[/itex] be the same set [itex]G_i[/itex] if [itex]G_i[/itex] is infinite and we let [itex]S_i[/itex] be denumerable if [itex]G_i[/itex] is finite. We let [itex]S[/itex] be a set having the same cardinality as the set-theoretic disjoint union of the sets [itex]S_i[/itex] (i.e. their coproduct in Set). We let [itex]\Gamma[/itex] be the set of group structures on [itex]S[/itex] , and for each [itex]\gamma\in\Gamma[/itex] we let [itex]\Phi_{\gamma}[/itex] be the set of all families of homomorphisms [itex]\varphi=\{\varphi_i\colon G_i\to S_{\gamma}\}[/itex] [Aside: I interpret this [itex]\varphi[/itex] with an indexing set [itex]i\in{I}[/itex] on [itex]\{\varphi_i\colon G_i\to S_{\gamma}\}[/itex] which I believe Lang dropped for convenience. Additionally, [itex]S_{\gamma}[/itex] is the group [itex]S[/itex] with group structure [itex]\gamma[/itex].]
Each pair [itex](S_{\gamma},\varphi)[/itex] where [itex]\varphi\in\Phi_{\gamma}[/itex], is then a group, using [itex]\varphi[/itex] merely as an index. We let [itex]F_0=\prod\limits_{\gamma\in\Gamma}\prod\limits_{\varphi\in\Phi_{\gamma}}(S_{\gamma},\varphi)[/itex], and for each [itex]i[/itex], we define a homomorphism [itex]f_i\colon G_i\to F_0[/itex] by prescribing the component of [itex]f_i[/itex] on each factor [itex](S_{\gamma},\varphi)[/itex] to be the same as that of [itex]\varphi_i[/itex].
Let now [itex]g=\{g_i\colon G_i\to G\}[/itex] be a family of homomorphisms. Replacing [itex]G[/itex] if necessary by the subgroup generated by the images of the [itex]g_i[/itex], we see that [itex]\left|G\right|\le\left|S\right|[/itex], because each element of [itex]G[/itex] is a finite product of elements in these images. Embedding [itex]G[/itex] as a factor in a product [itex]G\times S_{\gamma}[/itex] for some [itex]\gamma[/itex], we may assume that [itex]\left|G\right|=\left|S\right|[/itex]. [Non sequitur a la Lang?]
There exists a homomorphisms [itex]g_{*}\colon F_0\to G[/itex] such that [itex]g_{*}\circ f_i=g_i[/itex] for all [itex]i[/itex]. Indeed, we may assume without loss of generality that [itex]G=S_{\gamma}[/itex] for some [itex]\gamma[/itex] and that [itex]g=\psi[/itex] for some [itex]\psi\in\Phi_{\gamma}[/itex]. We let [itex]g_{*}[/itex] be the projection of [itex]F_0[/itex] on the factor [itex](S_{\gamma},\psi)[/itex].
Let [itex]F[/itex] be the subgroup of [itex]F_0[/itex] generated by the union of the images of the maps [itex]f_i[/itex] for all [itex]i[/itex]. The restriction of [itex]g_{*}[/itex] to [itex]F[/itex] is the unique homomorphism satisfying [itex]f_i\circ g_{*}=g_i[/itex] for all [itex]i[/itex]. We have thus constructed our universal object.Here are my questions related to the chunks of underlines.1.
Is there any motivation for choosing [itex]S[/itex] as the disjoint union of the [itex]S_i[/itex]'s? Could someone help elucidate that?
2.
Is there any reason for replacing [itex]G[/itex] by the subgroup generated by the union of the images of the [itex]g_i[/itex]'s -- that's how I interpret that phrase Lang has written there.
3.
What the hell is happening with embedding [itex]G[/itex] into [itex]G\times S_{\gamma}[/itex]? For instance, take [itex]G=\{0\}[/itex]; then it is clear that [itex]S[/itex] is never of the same cardinality as [itex]G[/itex], contradicting whatever Lang has said, as far as (more like 'if') I understand him.
4.
The 4th underline chunk is directly related to the 3rd.
5.
Again, any words of wisdom to offer about the thought process behind choosing the union of the images of the [itex]f_i[/itex]'s to generate [itex]F[/itex]?
The Attempt at a Solution
[/B]
I generally bang my head against a wall whenever I attempt to read this section and there are so many notes in the margins of my textbook that I haven't attempted ameliorating this by constructing my own such object. I'm almost afraid to try as some of the worst universal objects can be a daunting task to come up with. However, I suspect the object Lang is going for should enjoy the property that any group of the right size should be able to embed in it, similar to the construction of his Proposition 12.1.
I apologize for everything that's going on in this question, but I haven't been able to figure it out all day and I'm generally displeased with the style of writing in this section.