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## Month: June, 2014

### A short note about short exact sequences.

We will discuss short exact sequences here. They are of the form $A\xrightarrow{f} B\xrightarrow{g} C$, where im $f$= ker $g$.

Example: $\Bbb{Z}\xrightarrow{f}\Bbb{Q}\xrightarrow{g}\Bbb{R}$, such that $f(a)=a$ and $g(b)=b+\Bbb{Z}$.

What is the point of such a construction? Is there any way to visualize this? Whatever the image of $f$, you want to completely obliterate it. Leave no trace of it. Or maybe you want to check how similar the elements of the co-domain are to the image.

What if you have a mapping of the sort $0\to A\xrightarrow{\alpha} B\xrightarrow{\beta} C\to 0$? Assume that $0$ maps only to $0$. Also, if $A,B$ and $C$ are modules, $0$ would be the additive identity. The whole of $C$ maps to $0$. Hence, the kernel is the whole of $C$. Which means the im $\beta$ is the whole of $C$; i.e. $\beta$ is surjective. Also, $\alpha$ is injective, as its kernel is precisely $0$.

What if we had $0\to S\xrightarrow{\alpha} A\xrightarrow{\beta} B\xrightarrow{\gamma} C\to 0$? Here $\gamma$ is surjective, and $\alpha$ is injective. Can’t say much about $\beta$.

### Atiyah-Macdonald Part II

Let me start by first talking about the proof of the fact that the intersection of all prime ideals in a ring is the nilradical. The proof is constructive, and hence perhaps non-trivial. I will attempt to generalize the methodology of the proof.

This proof deals with creating a sort of border around the number objects that satisfy a certain property. Then it takes two external objects (that clearly don’t satisfy the property), and gets a third object. Now this third object may or may not lie inside the order. It is easily proven that it does not. Hence, _theorem_

Also, the mechanism of the proof could be used to prove that the maximal ideal containing all odd numbers is prime. The maximal ideal containing all even numbers is prime. The maximal ideal containing all powers of 2 is prime, etc.

12. Let $a\in R$ be an idempotent element. As $(a)$ is a proper ideal of $R$, it is contained within $M$, which is the unique maximal ideal. Hence, as $a$ is part of the Jacobson ring, $1-a$ is a unit. Now $(1-a)=(1-a^2)$. This implies that $a=0$, which is obviously a contradiction.

15. i) Easy

ii) Easy

iii) It is easy to prove. Note that this is not true only for the set of **prime** ideals containing $\{\bigcup E_i\}$. Let $W(A)$ be the set of ideals (any ideal) containing the set $A$. Then $W(\bigcup E_i)=\bigcap W(E_i)$. This is an important generalization.

iv) I have proved this in an earlier post. For what kinds of ideals do we have $V(a\cap b)=V(ab)$? Thinking about these things is what will give your thinking depth. The set of ideals which satisfy the following property: if $m^2\in I\implies m\in I$. Prime ideals and even radical ideals are vastly specialized examples of this very general condition.

For what kinds of ideals do we have $V(a)\cup V(b)=V(ab)$? Ideals which exhibit the following property: if $a\cap b\subseteq I$, then $a\subseteq I$ or $b\subseteq I$. Prime ideals are just one of the many types of ideals which may potentially fulfil this property. In fact, one may name a new class of ideals which are defined in such a way so as to fulfil this property.

Also, what kinds of sets mimic the behaviour of closed sets? Infinite intersections should be closed, finite unions should be closed, $\emptyset$ and the whole space $X$ (arbitrary name) should be closed, etc.

We see that $V(E)$ exhibits the properties of a closed set. How do we ensure that the infinite union of such closed sets is not closed? Refer to this question I asked on stackexchange- http://math.stackexchange.com/questions/846020/a-question-about-the-zariski-topology/846034#846034. The argument does not work for an infinite number of ideals because….well let me first give the argument for a finite number of ideals. Assume we have the ideals $a_1,a_2,\dots,a_n$, and $V(a_1a_2\dots a_n)$ does not contain $a_1,a_2,\dots,a_{n-1}$. Then it must contain $a_n$. Now if we have an infinite number of ideals, can we claim that $V$ does not contain all but one ideal? Yes. But can we prove that the ideal left aside is indeed contained within $V$? For that we need to deal with infinite products of elements. Such products are not defined. Hence, this argument does not translate to the infinite case.

Also note that the infinite union of closed sets may indeed be closed! Take the union of $[a_k,a_{k+1}]$, where $a_i$ is the $i^{th}$ natural number. However, the infinite union need not be closed. Take closed sets of the form $[\frac{1}{2^k},\frac{1}{2^{k-1}}]$ for example.

17. i) $X_f\cup X_g=(V(f)\cup V(g))'=(V(fg))'=X_{fg}$

A little word about **why** $V(f)\cup V(g)=V(fg)$ is true. If a prime ideal contains $fg$, and does not contain $f$, it HAS to contain $g$! This is also true if $f$ and $g$ are ideals, as mentioned in an earlier problem.

ii) If every prime ideal contains $f$, then $f$ has to be nilpotent. If $f$ is not nilpotent, there is a prime ideal which does not contain it, as the intersection of all prime ideals is the nilradical.

iii) If no prime ideal contains $f$, then $f$ is a unit. This is because if $f$ is not a unit, then $(f)$ is a proper ideal, and hence contained inside some maximal ideal. Every such maximal ideal is prime.

A little detour here. Why is every maximal ideal prime? Let $ab\in M$, where $M$ is maximal, and $a\notin M$. Then $as+m=1$, where $m\in M$ and $s\in R$ ($R$ is the commutative ring under consideration). Now multiply by $b$ to get $abs+mb=b$. Note that $abs$ and $mb$, both are in $M$, and $M$ is closed under addition (like every other ideal). Hence, $b\in M$.

iv) If $X_f=X_g$, then $V(f)=V(g)$. Now every ideal (not just prime ideals) containing $f$ will also contain $(f)$. Also, every prime ideal containing $(f)$ will also contain $r((f))$. Is that all? Will the intersection not contain any other element? Yes, we’re sure that the intersection will not contain any other element. Which brings me to my proof of this fact (not mentioned in Atiyah-Macdonald).

Let us consider an ideal containing $(f)$, which does not contain any power of $a$. Is this set nonempty? No, $(f)$ is one such ideal. Let us assume that the intersection of all prime ideals containing $f$ contains an element $a\notin r(f)$. It is easy to check that Zorn’s lemma can be used to create a maximum ideal not containing any power of $a$. It is also easy to prove that this maximum ideal is prime. Hence there is a prime ideal which does not contain $a$.

Now if $r((f))=r((g))$, then every prime ideal containing $f$ (and hence containing $r((f))$) will contain $r((g))$, and hence $g$. Hence $V(f)\subseteq V(g)$. We may similarly prove $V(g)\subseteq V(f)$.

v) The first thing to note here is that $V(f,g)=V(f)\cap V(g)$. Also: let $\{X_f,X_g,X_h,\dots\}$ be an open covering of $X$. Then $V(f)\cap V(g)\cap V(h)\cap\dots$ is empty. Note that the infinite intersection of closed sets is again a closed set. Also, that closed set is $V(f,g,h,\dots)$. If there is no prime ideal which contains ALL these elements, then there must be a finite number of them which add up to give $1$. Let those elements be $a_1,a_2,a_3,\dots a_n$. Then $X_{a_1},X_{a_2},\dots X_{a_n}$ also covers $X$.

vi) Let $X_f$ have a covering $\{X_m,X_n,\dots\}$. Then $V(m)\cap V(n)\cap\dots$ contains no ideal from $X_f$. Hence every ideal it contains contains $f$. In other words, $V(m,n,\dots)$ contains $f$. If that is true, then a finite number of them must add up to form $f$. Hence, the intersection of a finite number of $V(a)$ will lie completely inside $V(f)$. Taking their complements will give a finite cover of $X_f$.

What really is the gist of this argument? When we’re talking about an open set $X_f$, we’re really just talking about its complement, or the prime ideals which contain $f$. When we’re talking about an open cover, we’re really just talking about the intersection of the prime ideals in the complement. Hence, when one is a subset of another, we have a useful result. All prime ideals which contain these many elements also contain $f$…something like that. At the end of the day, all we’re doing is talking about various prime ideals containing certain elements, and what we can extract from that.

What if we had an open set of the form $X_f\cup X_g$? Then the complement is $V(f,g)$. If $X_f\cup X_g$ has an open cover, then there are finite expressions in $V(f,g)$ that are equal to $f$ and $g$. Let $a_1+a_2+\dots a_m=f$ and $b_1+b_2+\dots b_m=g$. Then taking taking complement of $V(a_1,a_2,\dots a_m,b_1,b_2,\dots b_n)$ gives an open cover of $X_f\cup X_g$.

vii)

18. i) The set is closed if it contains all ideals containing a certain element, say $f$. If $x$ was not maximal, then we could have made it bigger, and that bigger ideal would still contain $f$.

ii) This is by definition. It can be rather misguiding that there is no topological “proof” for this. For example, $\overline\{a,b,c\}=V(a,b,c)$ for any $a,b,c$ in commutative ring $R$.

iii) $\overline{\{x\}}$ is the set of all prime ideals containing $p_x$. Hence if $y\in\overline{\{x\}}$, then $p_y$ must contain $p_x$.

iv) Let us take two prime ideals $m$ and $n$. Also, suppose that $X_m$ contains $X_n$. This implies that $n$

### Solutions to some problems from Chapter 1 in Atiyah-Macdonald. And more importantly, insights and generalizations

If $x\in J$, where $J$ is the Jacobian ideal of a ring $R$, then $1-xy$ is a unit for all $y\in R$.

$1-xy$ looks like a particularly arbitrary expression. Let us break it down. $xy$ for all $y\in R$ is an alternate representation for $xR$. All maximal ideals contain $xR$. Now let us suppose that $1-xy$ is not a unit. Then $(1-xy)$ is contained in SOME maximal ideal (possibly not all). Let one of the maximal ideals $1-xy$ is contained in be $M$. Now $M$ also contains $xR$! Hence, that maximal ideal contains $1$, which is a contradiction.

Generalizing this, $a+xy$ should be a unit for any unit $a$ and for all $y\in R$.

Now the converse. Prove that $(1-xy)$ is a unit- $x$ belongs to the Jacobson radical. Suppose there is a maximal ideal that $x$ does not belong to. Let that ideal be $T$. Then $\exists t\in T$ such that $xm+tr=1$. There $1-xm=tr$. Now according to the condition stated above, $tr$ should be a unit. However, if that were true, $T$ wouldn’t be a maximal ideal. Hence contradiction. Actually this is true for the general expression $a-xy$, where $a$ is any unit in commutative ring $A$.

Proofs are for the verification of mathematical fact. They are not necessarily intuitive. They rarely offer deep glimpses into mathematical structures. How to “see” that $1-xy$ is a unit $\implies x\in J$? Assume that $I$ is an ideal of $R$ which does not contain $x$. Then $x$ can be added to it to create a bigger ideal. Hence it is contained in all maximal ideals. How can one be sure that the addition of $x$ will not create a unit within the ideal? It is not like the addition of $1$ is the only thing that can create a unit. There are very many ways of creating a unit- by the addition of ANY element in the ideal. Hence, the fact that $1-xy$ is a unit for ALL $y$ is an important condition for $x$ to be present in all maximal ideals. I unfortunately cannot offer a way to “see” this fact as of now. I can only stress on relevant facts.

Now I move on to solving problems.

1. It is easy to show that $(1+x)$ is a unit if $x$ is nilpotent. Is there a generalization? Yes. That is the next question. Any power of a unit is also a unit, like any power of a nilpotent is nilpotent. What about the product of a unit and nilpotent? In a commutative ring, it will obviously be nilpotent.

Let us generalize this. Any power of a nilpotent is a nilpotent. Any power of a unit is a unit. The product of a unit and nilpotent is nilpotent. The sum of a unit and nilpotent is a unit. Looking at things in some generality always makes me feel better.

2. i) Let $f(x)=a_0+a_1x+a_2x^2+\dots+a_nx^n$. If it is a unit, then there exists a polynomial $g(x)=b_0+\dots + b_mx^m$ such that $f(x)g(x)=1$ for all $x$. The condition for this is $a_0b_0=1$ and $a_nb_m=0,a_{n-1}b_m+a_nb_{m-1}=0$, etc. I’ve tried hard to prove it through a method other than the one mentioned in the book, but failed. I am used to only solving linear equations through substitution. Is this not substitution? No. Assuming that the linear equations are accurate, we can do a variety of things. We could have added them up and equated the sum to $1$. We could have multiplied all of the equations and equated the product to $0$. Substitution is just one of the many things we could have done. I need a broader perspective on solving linear equations.

Also note that $b_0$ is a unit too and $b_1,b_2,\dots b_m$ are nilpotent.

Another thing to note is that if $a+b$ is a unit, and $b$ is nilpotent, then $a$ is a unit. This is easy to prove. Let $a+b=c$, where $c$ is a unit. Then $a=c-b$, which is a unit by problem 1.

Proving the converse is easy. Use multinomial theorem.

ii) Similar to i.

iii) and iv) Done

3. Assume that all the polynomials are in $x_1$. If $f$ is a unit, all coefficients except for the constant are nilpotent. All those coefficients are polynomials in $\{x_2,x_3,\dots,x_n\}$. Proceed in this manner. This stuff is easy!

4. The general trick is to take an element $f$ of the Jacobson ideal, assume it is not nilpotent, take the unit $1-fy$, and put $y=x$. It is easy to see that $y$ can be ANY multiple of $x$. Also, $f$ need not be a polynomial. It can also be a constant element of the ring. This trick works even then.
In what kinds of rings are the Jacobson radical and the nilradical equal? It is known that the nilradical is always a subset of the Jacobson radical. It is difficult to comment as of now when the Jacobson ideal is in general equal to the nilradical. Here $A[x]$ is a particular ring with very specific properties for nilpotents. Other rings may have similar properties for nilpotents. What can be said is that for the properties that $1-fy$ must possess to be a unit for all $y$, those properties must make $f$ a nilpotent element, amongst other things that different values of $y$ may signify. Hence the Jacobson radial is likely to be a proper superset of the nilradical in most rings, as these are pretty specialized conditions.

5. i) Let the inverse of $f$ be $g$. Equate coefficients to $0$, and find suitable coefficients for $g$. Now you may wonder why this does not work for the finite degree case (problem 2). This is because after a finite number of steps (determining coefficients of $g$), you arrive upon a contradiction. Here, you can construct the power series $g$ from the coefficients determined, and see that multiplication happens perfectly, giving $1$. In the case of finite degree polynomials, if you construct the polynomial with coefficients derived in this manner, you will meet a contradiction.

ii) Mostly hinges on the fact that if $a,b$ are nilpotents, then so is $b-a$. There’s not a lot of coefficient hunting here. Only inductive reasoning. The converse does not HAVE to be true, going by the argument I put forward in problem 2. But I will still have to check Chapter 7 Exercise 2.

iii) Easy

6. Well I owe my solution to this link- http://asgarli.wordpress.com/2013/04/22/idempotent-elements-outside-the-nilradical/. However, I think my solution is slightly more straightforward. Let us assume that $J\neq N$, where $J$ is the Jacobson radical and $N$ is the nilradical. Then there exists an idempotent element in $J$. Let that element be $e$. Then $1-ey$ is a unit for all $y\in R$. In particular, $1-e^2=(1+e)(1-e)=1-e$ is a unit. We get $1+e=1$. Hence $e=0$, which is a contradiction as $e$ is assumed to be non-zero. Hence the Jacobson radical cannot contain any element apart from nilpotent elements.

7. Let $P$ be a prime ideal. Assume $P$ is not maximal. Then there exists $a\in R$ such that $(P,a)$ is not equal to $R$. Now $a^n=a$ for some $n\in\Bbb{N}$. Hence $a^n-a=0$. Therefore $a(a^{n-1}-a)=0$. If we take $R/P$, we still have $(a+P)(a^{n-1}-1+P)=0$. Seeing as this is an integral domain, and knowing that $a\neq 0$, we get $a^{n-1}=1+p$, where $p\in P$. Now follow closely: when we add $a$ to $P$, we also add $a^{n-1}$ to it. Hence, we’re effectively adding $1$ to $P$. This is a contradiction.

Alternate proofs can be found elsewhere. A common proof is to say that $R/P$ is maximal, as $x$ has $x^{n-2}$ is the multiplicative inverse of $x$. I just wanted to add a slightly different flavour to it.

8. Zorn’s lemma. A clever chain argument is used here. Try for yourself.

9. This is equivalent to proving that the intersection of all prime ideals containing $a$ is $r(a)$. Atiyah-Macdonald gives a concise proof: Consider the ring $R/a$. The inverses of all prime ideals containing $a$ will be prime ideals containing $a$ (inverse obviously of a mapping from $R$ to $R/a$). The nilradical will be the intersection of all prime ideals in $R/a$. Check that the inverse of this nilradical will indeed include the whole of $r(a)$.

10. $i)\to ii)$ Solve for yourself. Remember that every maximal ideal is prime.

$i)\to iii)$ The lone prime ideal is the maximal ideal, which is also equal to the nilradical.

$ii)\to i)$ Let us take the nilradical as the starting set. Every prime ideal can be built on it. Every prime ideal has to contain it. Draw pictures to give yourself a visual understanding of this!! However, we cannot build a bigger prime ideal, starting out from the nilradical, as all external elements are units! Hence there is only one prime ideal.

11. i) $2x=x+x=2x^2=2x^2+2x$. Hence, $2x=0$. How many ways are there of solving such a question? What are the kind of strategies one should opt for? Is hit and miss the only way? In general, we can construct $nx=(x+x+\dots+x)=(x^2+x^2+\dots x^2)=(x+x)^2+x^2+x^2+\dots+x^2=\dots=(x+x+\dots+x)^2$

ii) Proving every prime ideal is maximal is easy. Refer to problem 7. However, we notice that in a lot of the last problems we’ve had prime ideals equal to maximal ideals. What is the general case? When are prime ideals equal to maximal ideals? Problem 7 says one possibility is when for all $x\in R$, $x^n=x$ for some $n\in\Bbb{N}$. The $n$ doesn’t have to be the same for different elements. But what if $x^n=2x$? or $3x$? Does it still work? It does not always work. BUT IT WORKS FOR $x^n=ax$, WHERE $a$ IS A UNIT!! Check if for yourself. Here is the generalization we were waiting for!!

Proving that $R/p$ is a field with two elements is easy if you walk down the same path as mentioned above. For $a\in R/p$, we have $a^2-a=0$. Hence $a(a-1)=0$. This shows that either $a\in p$ or $a\in a+p$.

Note: Constructing ring modulo prime ideal seems to be a source of many useful discoveries. When is this technique useful? It is useful when we’re considering elements with respect to the prime ideal- namely whether they’re inside or outside it. It is unlikely to be useful in a lot of other places.

iii) This involves the creation of a generator. One option is $x+y+xy$. Are there other generators?

More importantly, in what kinds of cases is every finitely generated ideal principal? One possibility is when the Euclidean algorithm is valid, and it is possible to determine the gcd of the generators. Another is when rings have special relations, which allow the creation of generators by multiplication with a single element. For example, if the ring had the relation that $xy=0$, and $x^2=x,y^2=y$, then $\langle x,y\rangle=\langle x+y\rangle$. If the ring had a relation $x^2=x,y^2=y,nxy=0$, then $\langle x,y\rangle=\langle x+y+(n-1)xy\rangle$

### Localization of a ring

So what exactly is the localization of a ring?

It is creating a field-imitation (and NOT necessarily a field with multiplicative inverses) for every ring. It includes the creation of multiplicative inverse- imitations (for elements that are not zero-divisors).

How does it do this? It apes the steps taken to create a field of fractions from an integral domain. A good description can be found on pg. 60 of Watkins’ “Topics in Commutative Ring Theory”, First Edition.

I would like to concentrate on one small point. Why is it that $\frac{a}{b}=\frac{c}{d}\implies t(ad-bc)=0$ where $b,d,t$ belong to the same multiplicative system? This description does not exactly ape the condition for integral domains: $\frac{a}{b}=\frac{c}{d}\implies (ad-bc)=0$. Simply put, why the extra $t$??

Because we have no means of isolating a regular element that is a common factor!! In every case, including that of integral domains, we get something like $h(ad-bc)=0$, where $h\in R$. Then we say $h$ is not a zero divisor. Hence, $ad-bc=0$. Here, we can’t say anything about $h$. Hence, the condition here is that $\frac{a}{b}=\frac{c}{d}\implies h(ad-bc)=0$.

Now where does the “multiplicative system” part come in?? That part seems fairly arbitrary, doesn’t it? How does it help prove transitivity? Because you’re going to get something of the form $h(af-be)$ anyway! Hence, if you didn’t add the $t$, how would you justify the $h$? Simple?

But why “multiplicative system”? Why not a simple $\frac{a}{b}=\frac{c}{d}\implies t(ad-bc)=0$ for some $t\in R$? Mainly because this allows us to localize $R$ with respect to different multiplicative systems within the ring. It helps us to generalize. We can choose the denominators and $t's$ (as in $t(ad-bc)$) from subsets of the ring, or the whole ring itself. It is the properties of these subsets that we will explore in the coming paragraphs.

Say we localize with respect to multiplicative system  $E$ of ring $R$. Then we choose all denominators from $E$. We need to choose denominators from the same multiplicative system because the way fraction multiplication is defined: we want multiplicative closure.

Why choose $t$ in $t(ad-bc)$ from the same multiplicative system as the denominators too? Why choose $t$ from a multiplicative system at all? We have to choose $t$ from the same multiplicative system because soon we start having products of $t's$, and we don’t want them to be from outside of the set we’re choosing the $t's$ from. This multiplicative system is the same as that of the denominators because denominators are also multiplied to the $t's$. Hence, we want algebraic closure.

I’ve always found the construction of the quotient field of a domain very arduous and time-consuming. Most people get lost somewhere in the proof. I am going to try and make it more transparent.

What are we trying to do? We’re trying to convert a ring into a field (please forgive my language). How are we doing it? We’re creating fractions out of numbers. Hence, consider the set of two-tuples $(a,b)$, where $a,b\in R$. Here, $R$ is the ring.

Why do we need $R$ to be an integral domain? Let us break this down

1. We (sort of) know that $(a,b)=\frac{a}{b}$. Now consider $\frac{a}{b}\times\frac{c}{d}=\frac{ac}{bd}$. By definition, $b,c\neq 0$. However, what if $bc=0$? Hence we can’t have zero-divisors.

2. We are making equivalence classes of elements, where $(a,b)\equiv (c,d)$ if $ad=bc$. This relationship is transitive only if the elements of the ring are commutative. Hence, we need commutativity. Construct a counter-example with $D_{2n}$ (which is non-commutative).

3. We also need $1\in R$, as we need a multiplicative unit in the field. A field always contains the multiplicative identity.

A ring with the three properties listed above is an integral domain. Hence, we need an integral domain.

Now just define addition and multiplication, and ensure that the operations are well-defined.

The thing that is tricky about this proof (for beginners) is that the construction of equivalence sets and verifying well-defined-ness is not intuitive. People just generally want to define addition and multiplication in the obvious way, and be done with it. As long as one remembers that checking whether the operations are well-define or not is important, one is fine. Note that if $a\equiv b$ and $c\equiv d$, and $a+c\not\equiv b+d$, then the operation is nonsensical. To make it clearer for the beginner, this is equivalent to saying $x+y\neq x+y$.

### Developing new axioms for Elementary Geometry

I have always been bad at geometry. Always.

How does one develop geometric intuition? Let us talk about the fundamental building blocks of Geometry- Congruency. I must confess I never really understood HOW the whole of geometry is motivated by a seemingly useless and, let’s face it, mysterious concept of Congruency.

Two figures which are exactly the same under rigid motion are said to be Congruent. Congruency has always been only about triangles in school. This is an arbitrary restriction, and should be changed (bad teaching at the school level is what drives most people away from Geometry). Congruency conditions can easily be developed for squares, hexagons, dodecagons, you name it.

For example, if two squares have the same side length, they are the same. Simple.

Quadrilaterals- If three sides and three angles of two quadrilaterals are the same, then the two are the same quadrilateral.

Seeing Geometry from this generality sure makes the subject more comprehensible. Even today, whenever I see a geometrical figure, I just get irritated as I start believing that an endless bout of angle chasing and congruency proofs (of triangles) will follow. Now this bout is more motivated than before.

1. Distances and angles- Angles and distances are thought to be different measures in geometry. The concept of distance is fairly intuitive (to me). I have always felt that angles are a non-intuitive and arbitrary construct, and they really are just a way of representing distance. I am going to go off on a tangent and give a distance-centric definition of angle.

Let us take two lines (or any curves), and consider distances between points. There are many reasonable ways of dong this. One way is: take a point $p_1$ on $L_1$, and drop a perpendicular on $L_2$. Let the point of intersection be $p_2$. Now consider $d(p_1,p_2)$. After we consider the distances between all such pairs of points, we can assign an “angle” to the pair of lines. But how does this assignment work? There are multiple reasonable ways, again. We could consider distances only between a reference point on $L_1$ and the point on the perpendicular to $L_2$, etc. We could also see the general trend of increasing or decreasing distances between points. Whatever. Depending upon these distances, the assignment of the angle between the two lines is done. Hence, angles have a totally distance- motivated definition.

2. Parallel lines- I want to give an intuitive feel for how parallel lines are constructed. Take two intersecting lines. Now move one line (along the base line) in such a way that the distances between points remain the same (find a reasonable way to parse this statement). Then the initial line and the moved line are parallel, and the angle between the moved line and base line is the same as that between the initial line and base line (as the distances remain the same).

3. Tangent- An imaginary construct. A line that touches a curve only at one point. Impossible to construct by hand. But then again, a point would be an imaginary construct too. We’re talking about limits here anyway.

4. Area and volume- The formulae are only indicative of “how much” matter there is (or capacity for matter); both area and volume are proportional to this “how much”. As the formulae are pretty random, a separation of units does the trick- we can only compare two volumes, and not an area and a volume, although technically we SHOULD be able to compare both as both are representative of the amount of matter (or capacity of a vessel, etc). However, the volume and area formulae are certainly getting something right. If I take two arbitrary vessels of different shapes but same volume (as calculated by the formula), they will contain the exact same amount of water!. Hence, the current formula is clearly proportional to the amount of matter (if such a concept can ever be fully fleshed out in the future).

### VSRP 1: On V(a)

So what exactly are prime ideals? They are ideals such that $ab\in I\implies a\in I$ or $b\in I$.

Let $a\in R$, where $R$ is a commutative ring. How is the Zariski topology motivated?

Let $V(a)$ be the set of all prime ideals which contain $a$. All ideals that contain $a$ contain the whole of $aR$. Hence all members of $V(a)$ also contain the whole of $aR$.

Now take $V(a,b)$, where $a,b$ is any arbitrary two-element set in $R$. Clearly $V(a,b)= V(a)\cap V(b)$. Note that for both these properties, there is nothing special that is true only for prime ideals and not for any other ideal containing the elements.

Now take $V(r(aR))$, where $aR$ is the ideal generated by the element $a$. Here $r(aR)$ is the radical of $aR$. It can be proved that $V(aR)=V(a)=V(r(aR))$. The inclusion of $V(r(aR))$ in the equality relation $V(a)=V(aR)$ is what sets apart prime ideals containing $a$ from any random ideal containing $a$.

Now we will go one step further. Let $\{P_k\}=V(a)$ be the prime ideals containing $a$. We will prove that $\bigcap\{P_k\}=r(aR)$. It is known that the nilradical of $R$ is the intersection of all prime ideals in it. Now consider prime ideals in $R/(a)$, where $(a)=aR$. The intersection of all prime ideals in $R/(a)$ will again be the nilradical of $R/(a)$. The inverse of all such prime ideals will be prime ideals containing $(a)$ in $R$. The rest of the argument is easy to see from here.

I have been been part of the VSRP program for about 10 days. I have solved some problems from Atiyah-Macdonald and some from a couple of other books. I have also brushed a little of topology and other parts of Algebra.

In spite of having solved problems on prime ideals and the Zariski Topology, there is nothing that I am taking away from doing these exercises. I only have a vague picture of the underlying mathematical structure, but nothing much more. Hence unless I am really intellectually handicapped, I feel the practice (followed by most places across the world) of teaching Mathematics by solving problems is inherently flawed. The focus I feel is more on somehow being able to solve the problem, and then moving on to the next one. There is little reward to maybe generalize the problem and think about the motivation behind the problem- the generality that the problem is hiding in the form of a “trick”. I don’t think I am a big fan of this method.

I think being able to describe a mathematical structure in words is an important component of the learning process. Hence, I will continue blogging.

### TIFR VSRP

I have been at TIFR for a week. Some thoughts:

The emphasis is clearly on “what book have you read? I followed this book. Have you attempted the exercises from this book?” This is similar to what I saw at ISI, Kolkata. I did not focus on problem solving while learning Mathematics on my own. My emphasis was on extrapolating definitions into unknown territory. I wanted to get a feel for the definitions. This proved to be a handicap in some ways as I could never quite get a hang of the techniques and machinery which would later help me understand deeper problems. Solving problems would have given me a sense of direction: it would give me a feel for which direction the mathematical community has taken after formulating these definitions (actually in most cases, before formulating those definitions).

However, I feel not solving problems has also been a major advantage. Now I know there is a world outside of problem solving, and hence aligning your outlook and interests with the rest of the mathematical community. I have the benefit of an outsider’s view. I have tried to go where no one else thought of going. If everything works out well, I might also have an insider’s view soon (after having worked through tons of problems in classic textbooks, of course). The future doesn’t look as bleak as I initially thought it would.