# Question on Quotient group

The basic concept of Quotient Group is often a confusing thing for me,I mean can any one tell the intuitive concept and the necessity of the Quotient group, I thought that it would be nice to ask as any basic undergraduate can learn the intuition seeing the question. My Question is :

1. Why is the name Quotient Group kept,normally in the case of division ,let us take the example of $\large \frac{16}{4}$ the Quotient of the Division is '$4$' which means that there are four '$4$'s in $16$ ,I mean we can find only $4$ elements with value $4$

So how can we apply the same logic in the case of Quotient Groups,like consider the Group $A$ and normal subgroup $B$ of $A$, So if $A/B$ refers to "Quotient group" ,then does it mean :

Are we finding how many copies of $B$ are present in $A$?? ,like in the case of normal division ,or is it something different ??

I understood the Notion of Cosets and Quotient Groups,but I want a different Perspective to add Color to the concept,Can anyone tell me the necessity and background for the invention of Quotient Groups ??

Note:I tried my level best in formatting and typing with proper protocol,if in case ,any errors still persist,I beg everyone to explain the reason of their downvote (if any),so that I can rectify myself,Thank you.

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You're finding how many cosets of $B$ are present in $A$. The quotient group is the group of cosets. – Michael Hardy Oct 1 '11 at 16:54
One should consider the fact that the quotient group $A/B$ was originally sometimes called difference group and denoted $A-B$ (although in the abelian case only, I guess) – Mariano Suárez-Alvarez Oct 1 '11 at 16:56
@iyengar, a minor pro tip: This is a fairly egalitarian place, and sir'ing everyone you speak to will tend to make them slightly twitchy. We all try our best not to let that show, because it's really irrelevant, but doing so takes some mental energy that could be used to provide a better answer instead. (Using some full stops now and then rather than commas wouldn't hurt either, but that's easier to ignore than obsequious mannerisms). – Henning Makholm Oct 1 '11 at 18:56
If the people you want to respect are feeling uncomfortable with "sir", you are doing them a favor by not using "sir" to address them. – Soarer Oct 5 '11 at 5:41
@iyengar You might also be interested to know that your obstinate insistence on ignoring spacing rules had the effect that I wasn't notified of your comment, AlexB not being my nick, nor part of my nick on math.stackexchange. – Alex B. Oct 7 '11 at 6:39

Often I think of a quotient group in terms of (loss of) information. When we move from a group to its quotient group we lose some information about the identity of the elements. For example, when we map an element of the additive group of integers $\mathbf{Z}$ to the quotient group $\mathbf{Z}/10\mathbf{Z}$ we lose the information of all the other digits save the least significant one. In other words after moving down to the quotient group we can no longer tell the difference between 9, 999, or 314159. In this sense we then equate 9 with 99 et cetera

Why would we want do this, as it amounts to loss of information? Well, there are several reasons. Sometimes we are only really interested in the residual information. For example, when we study the set of numbers of the form $a+b\root 3\of 2+c\root 3\of 4$, where $a,b,c$ are integers, and we want to start adding, subtracting and multiplying them, we quickly notice that those operations are very similar to the corresponding operations involving polynomials $a+bx+cx^2$. The difference is that we are only interested in the value of the polynomial at a single point $x=\root 3\of 2$. This shows in the multiplication rule, because the polynomial $x^3$ takes the value $2$. In order to make this corresponds between polynomials and numbers more accurate we are forced to equate the polynomial $x^3-2$ with the polynomial $0$. This time we get a quotient ring instead of a quotient group (see algebra textbooks for such details), but the idea is that some things we have learned about polynomial algebra will carry over to our set of numbers, and that gives us the benefit of economy of thinking. We don't need to relearn everything from scratch, if the next time we are interested in $\root 3\of 3$ instead.

Sometimes quotient groups are forced upon us. We are not in possession of all the information. A simple example is the following. Assume that somebody is counting coins, but the only counting aid available to him is a light switch. Every time he tallies one more coin he will toggle the light switch: lit, dark, lit, dark,... He may or may not be able to keep track of the actual tally, but if somebody else comes to the room, or the tallyman gets confused, the status of the light switch will only tell whether an odd or an even number of coins have been counted, i.e. we have moved from the group $\mathbf{Z}$ to the quotient group $\mathbf{Z}/2\mathbf{Z}$. Another very common quotient group in mathematics is used to decribe an angle of rotation. Let's say that we are studying a planar object spinning about its center of mass. It may have completed God knows how many full revolutions, but when we enter the room and observe its position, we have no way of knowing anything else but the current direction pointed at by, say, a small arrow somebody painted on the object for this purpose. A full revolution corresponds to an angle of rotation $2\pi$, so the total angle of rotation will have an uncertainty that can be any integer multiple of $2\pi$. In other words, we can only see an element of the quotient group $\mathbf{R}/2\pi\mathbf{Z}$, not an element of $\mathbf{R}$.

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very precise and good and deep explanation,thank you sir, – Iyengar Oct 1 '11 at 17:51
Also, rather than "lose information", one may also sometimes view it as "ignoring information which is not very relevant for some purposes". Moding out by the squares/multiples of $2$ keeps information about parity, which may be all that is needed in one particular instance (so that we are ignoring information which is not relevant but may be obscuring what is "really going on" by sheer volume). – Arturo Magidin Oct 1 '11 at 20:47
@Arturo A good point! I was trying to include something like that when writing the paragraph on the theme only interested in the residual information, but forgot some of it and didn't quite manage to convey it all. The answer is long enough as it is. – Jyrki Lahtonen Oct 1 '11 at 21:04
@iyengar I won't. I'm not familiar with the Tate-Shafarevich group at all. You should spend a couple of years becoming fully conversant with the examples on this thread and other absolutely necessary background material before you even look at elliptic curves. Then you should study the elliptic curves for a couple of years: recognize the holes in your theoretical background and fill them. Only after that is there any chance that you would understand what the Tate-Shafarevich group is measuring. I don't, but my 30+ years of math have been mostly targeting other stuff. – Jyrki Lahtonen Oct 3 '11 at 7:02
@iyengar Sorry, if my previous comment sounded harsh. It was meant as a genuine advice. I really think that you should concentrate on studying and understandinf these simple concepts, and not obsess about Birch - Swinnerton-Dyer. – Jyrki Lahtonen Oct 3 '11 at 9:22

Quotient groups $A/B$ count cosets of $B$ inside $A$. The counting even works well with addition.

The Cartesian plane forms a group A, and a line through the origin is a subgroup B. The cosets of B inside A are all the parallel lines. How many are there?

Suppose B is the line: $$B = \{ (x,y) : y = 2x \}$$ or just B is y = 2x for short. Parallel lines are parameterised by their "intercept" b, the coset b + B is the line with slope 2 and intercept b. $$b+B = \{(x,y) : y = 2x+b\}$$ This means that there is exactly one coset for every real number. In some sense we have counted the parallel lines.

The way we count them even keeps track of addition. If I took a point on the line B, say (2,4), and added it to a point on 7 + B, say (3,13), then I get the point (5,17), which is on the line $$(0 + B) + (7 + B) = 7 + B.$$ If I add the point (3,7) on 1 + B to the point (8,20) on 4 + B, then I get the point (11,27) on $$(1+B) + (4+B) = 5 + B$$ This is just because if x = 11, then 2x is 22, and 27 is 2x+5.

If one wanted to be more precise, I suppose one should say (0,5) + B, since it should be an element of $A$ plus $B$, but just like $A/B \cong \mathbb{R}$, we only need one number here too.

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In a sense, the quotient group is indeed a measurement of how many copies of your normal subgroup are within the larger group. In the simple example of $\mathbb{Z}/3\mathbb{Z}$, the group has three elements: one for the subgroup $3\mathbb{Z}$ itself and one for each of its two cosets, which, if you were to plot them on a number line, for example, "look" the same as the original subgroup. And if you put the subgroup and its two cosets together, you get the whole group $\mathbb{Z}$. So in a sense the quotienting in this case tells you how many subsets resembling $3\mathbb{Z}$ are needed to break down $\mathbb{Z}$. The thing that makes this different from arithmetic division, of course, is the fact that the quotient is also a group--the group structure just comes from the way the "copies" of the subgroup interact with each other.

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 "but Sir,can you give me a different example rather than the congruence modulo,suppose can you tell me ,how does it work in the case of some other groups(other than cyclic groups),I mean consider $E(\mathbb{Q})/2E(\mathbb{Q})$ ,where $E(\mathbb{Q})$ is the Group of rational points on the curve,so how can you explain that Quotient group,and also the multiplicative group ${\mathbb{Q}}^{\times}/({\mathbb{Q}}^{\times})^2$,how can you explain the above groups ,i mean tell me the intuition,the Second one was the Multiplicative group ,which means that the $3$ and $\frac{12}{25}$ are similar – Iyengar Oct 1 '11 at 17:05

Two of the most basic concepts in mathematics, namely "Sets" and "Relations" are much useful to create new things from old. For example, "integers" can be constructed from natural numbers by putting an equivalence relation on the set $\mathbb{N}\times \mathbb{N}$ (see Concrete Abstract Algebra: Niels Lauritzen), rationals can be constructed from integers by putting an equivalence relation on $\mathbb{Z}\times \mathbb{Z}$; in some way reals, complex numbers can also be constructed. The only things we have used are "Sets and Equivalence Relation".

In this way, we can think of quotient group of a group $G$ as a set of equivalence classes of group $G$ with a natural binary operation, induced from that of $G$:

If $H$ is a subgroup of $G$, define an equivalence relation on $G$ by $x\sim y$ iff $x^{-1}y\in H$, denote equivalence class of $a$ by $[a]$. If $K$ is the set of equivalence classes of $G$, a natural way to define a binary operation on $K$ is to define $*\colon K \times K\rightarrow K$ by $[a]*[b]=[ab]$.

(So we have constructed a new set using equivalence relation and trying to put a binary on it) The necessary (and sufficient) for this map to be well defined (hence binary operation) is that $H$ should be normal subgroup of $G$. Once we convince for it, it is easy to verify other conditions in the definition of group. The new group is called as quotient group of $G$ by $H$ (denoted $G/H$)

Therefore, the quotient group of a group by normal subgroup is a set of equivalence of classes of $G$ with a natural binary operation, induced from $G$

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 thank you sir for the answer – Iyengar Oct 3 '11 at 13:22