# Category Archives: Set theory

## A countability argument

Here is a countability argument that I like because it relies on almost nothing. Let $A$ be a ring.

Theorem. If $A$ is countable, then the polynomial ring $A[X]$ is countable.

Proof. Since $A$ is countable, there is an injection $f:A\to\mathbb N$. Let $p_0 be prime numbers and consider the map

\begin{aligned}g:A[X]&\to\mathbb N\\ a_0+a_1X+\cdots+a_nX^n&\mapsto p_0^{f(a_0)}p_1^{f(a_1)}\cdots p_n^{f(a_n)}.\end{aligned}

By unique factorisation in $\mathbb N$ it follows that $g$ is an injection. Thus $A[X]$ is countable. $\square$

We can use this to prove in a rather simple manner that

Corollary 1. The set $\mathbb A$ of all algebraic numbers is countable.

Proof. It follows from the above that $\mathbb Z[X]$ is countable. Let $\alpha\in\mathbb A$ be a root of some minimal polynomial $f_\alpha\in\mathbb Z[X]$. We can assign to each $\alpha\in\mathbb A$ a unique element $f\in\mathbb Z[X]$ as follows: if $\alpha_1(=\alpha),\dots,\alpha_n$ are the zeros of $f_\alpha\in\mathbb Z[X]$, assign $jf_\alpha$ to $\alpha_j$. This gives an injection from $\mathbb A$ to $\mathbb Z[X]$, as desired. $\square$

More generally,

Corollary 2. A countable union of countable sets is countable.

Proof. Let $A_0,A_1,\dots$ be countable sets. Then there are injections $A_i\to X^i\mathbb Z$ for $i=0,1,\dots$. Hence we have an injection

$\displaystyle\bigcup_{i=0}^\infty A_i\to \bigcup_{i=0}^\infty X^i\mathbb Z\subseteq\mathbb Z[X]$,

showing that $\bigcup_{i=0}^\infty A_i$ is countable. $\square$

Filed under Set theory

## An application of Zorn’s lemma: Transcendence bases

Zorn’s lemma is a very useful result when it comes to dealing with an infinite collection of things. In ZFC set theory it is equivalent to the well-ordering theorem (every set can be well-ordered) and to the axiom of choice (a Cartesian product of non-empty sets is non-empty). I happened to use it a few days ago in proving the existence of transcendence bases, hence this post!

Zorn’s lemma. If every chain in a poset $P$ has an upper bound in $P$, then $P$ contains a maximal element.

Let $K/k$ be a field extension. We call a subset $S\subseteq K$ algebraically independent if for any $m\ge 0$ and $s_1,\dots,s_m\in S$, $p(s_1,\dots,s_m)=0$ implies $p=0$, where $p\in k[X_1,\dots,X_m]$ is a polynomial. A maximal (with respect to $\subseteq$) algebraically independent subset is called a transcendence base.

Theorem. Every field extension has a transcendence base.

Proof. If $K/k$ is algebraic, the transcendence base is the empty set. Suppose that $K/k$ is not algebraic. Let $\mathcal F$ be the family of all algebraically independent subsets $S\subseteq K$. By Zorn’s lemma, it suffices to show that every chain in $\mathcal F$ has an upper bound. Let $\mathcal C$ be a chain in $\mathcal F$, and let

$T=\displaystyle\bigcup_{S\in \mathcal C} S$.

It suffices to show that $T\in\mathcal F$, since then $T$ would be an upper bound for $\mathcal C$ in $\mathcal F$.

Let $P(m)$ be the statement:

If $t_1,\dots,t_m\in T$ are distinct, and $p(t_1,\dots, t_m)=0$ for some $p\in k[X_1,\dots,X_m]$, then $p=0$.

If $t_1\in T$, then $t_1\in S$ for some $S\in \mathcal C$. Since $\mathcal S$ is algebraically independent, $p(t_1)=0$ implies $p=0$ in $k[X_1]$. Thus $P(1)$ is true.

Suppose $P(m-1)$ is true. Let $t_1,\dots,t_m\in T$ be distinct such that $p(t_1,\dots,t_m)=0$ for some $p\in k[X_1,\dots,X_m]$. We can view $p(t_1,\dots,t_m)$ as a polynomial in $t_m$ with coefficients in $k[t_1,\dots,t_{m-1}]$, i.e. say

$\displaystyle p(t_1,\dots,t_m)=\sum_{i=0}^n p_i(t_1,\dots,t_{m-1})t_m^i=0$.

Since $P(1)$ is true, it folows that $p_i(t_1,\dots,t_{m-1})=0$ for each $i=0,1,\dots,n$. But then $p_i=0$ for each $i$ by our hypothesis. Thus $p=0$, implying $P(m)$ is true.

Therefore, by induction, $T$ is algebraically independent, i.e. $T\in\mathcal F$. $\square$

Filed under Algebra, Set theory

## Ordering in groups

This idea first came to my mind a long time ago when I was trying to generalise the application of the division algorithm. All I came up with is the following:

Proposition 0. Let $S$ be a subgroup of $(\mathbb R,+)$. If $S$ has a least positive element, then $S$ is cyclic. Otherwise $S$ has arbitrarily small positive elements.

A few days ago, in a Number Theory lecture, we proved that the minimal solution to Pell’s equation generates all solutions. It reminded me of the division algorithm again, so it was time again to try to generalise this, because now I have a few more tools in hand!

Let’s say $S$ is a set where the division algorithm applies. We definitely need some sort of partial order in $S$ to say that the ‘remainder’ must be ‘less than’ the ‘divisor’. We might want $S$ to be closed under some operation (so that we can repeatedly ‘subtract’ the ‘divisor’ from the ‘dividend’) and we also need an inverse operation (i.e. the ‘subtraction’).

So first of all, we need $S$ to be a poset. In addition, the closure and inverse operations suggest that we want $S$ to be a group. How should the order behave under the group operation? Clearly we want it to be compatible with the operation. We also probably want the inverse of a ‘positive’ element to be ‘negative’, and vice-versa. Do we need the group to be abelian? Maybe, but let’s not impose that condition yet.

So to put our ideas into work, let $(G,+)$ be a group with a partial order $\le$ such that for all $g,g_1,g_2\in G, g_1\le g_2\Rightarrow g+g_1\le g+g_2$ and $g_1+g\le g_2+g$. We say that an element $g\in G$ is positive if $0\le g$, where $0$ is the identity element of $G$. Define the positive cone of $G$ to be the set $G^+:=\{g\in G:0\le g\}$ of all positive elements.

Okay now we hopefully have all the necessary definitions in place. Let’s see if we can prove anything using these. The first thing that we want is probably: if $0\le g$, then $-g\le 0$. This follows easily: $0\le g$, add $-g$ to both sides and we are done. It works the other way around as well, so in fact we have proved:

Lemma 1. $0\le g\Leftrightarrow -g\le 0$.

That was good. We wanted the inverse of positive elements to be negative and it just followed from the definition. But we want more! So take a non-zero element $g\in G$. By Lemma 1 we can take $g$ to be positive without loss of generality. How about adding something to both sides of $0\le g$ again? Last time we added $-g$. We can add $0$, but that doesn’t change anything. So the only obvious choice left is to add $g$: $g\le g+g=2g$. Now what? Let’s add $g$ again! $g+g\le 2g+g$, i.e. $2g\le 3g$. Combining the last two gives $0\le g\le 2g\le 3g$. It follows by induction that $mg\le ng$ for all integers $0\le m\le n$ (note: here $ng=g+\dots+g$, $n$ times). This looks promising.

What about ‘negative’ elements? Note that by Lemma 1, $-g\le 0$. Adding $-g$ to both sides yields $-2g\le -g$, and so on. So we have another nice result:

Lemma 2. $mg\le ng$ for all integers $m\le n$ and $g\in G^+$.

It seems that this is all we can derive from our first principles. So let’s apply more restrictions on $G$. Let $a,b\in G$ be positive with $b\le a$. As in the division algorithm, let’s look at $a-b,a-2b,\dots$ etc. We want this sequence to stop as soon as $a-nb$ becomes negative. How do we do this? In other words, we want the set $\{a-nb:n\in\mathbb Z\}$ to have a least positive element. Did something just pop up in your mind? A set having a least element must have reminded you of something like… the well-ordering principle! So how about we impose the extra condition that $\le$ is a well-order on $G$? Well, that’s clearly absurd, because for any positive $g$ the set $\{ng:n\in\mathbb Z\}$ has no least element. How about least positive element then? In other words, let’s say $G^+$ is well-ordered under $\le$.

Now $G$ has quite a few nice properties: it is a group under $+$, $\le$ is an order on $G$ preserving $+$, and its positive cone is well-ordered. Let’s see if our ideas work now.

Let $\varepsilon$ be the least non-zero element in $G^+$ and $g\in G$ be any non-zero element. Without loss of generality, $0. Then $g\in G^+$ so $\varepsilon\le g$. Consider the elements $n\varepsilon$ for $n\in\mathbb Z$. We want $n\varepsilon\le g<(n+1)\varepsilon$ for some $n$. Can we achieve this? We certainly have $n\varepsilon\le g$ for $n=1$, so we need $g for some $n'$. By Lemma 2 $n'$ must be greater than $n$. How do we know that $n'$ exists?

Suppose it doesn’t. Then $n\varepsilon\le g$ for all $n\in\mathbb Z$ by totality (recall that a well-order is a total order) and Lemma 2. Then $0\le g+n\varepsilon\;\forall n$, so $g+n\varepsilon\in G^+\;\forall n$. Hence $\{g+n\varepsilon:n\in\mathbb Z\}\subset G^+$, so it has a least element $g+m\varepsilon$. Then $g+m\varepsilon\le g+n\varepsilon\;\forall n$ which implies $0\le n\varepsilon$ for all $n\in\mathbb Z$, a contradiction.

That was really good! Now we can take the maximal $n$ such that $n\varepsilon\le g$. Then $g<(n+1)\varepsilon$. Then $0\le g-n\varepsilon<\varepsilon$; the left inequality says $g-n\varepsilon\in G^+$, and the right inequality says $g-n\varepsilon<\varepsilon$. So $g-n\varepsilon=0$ and $g=n\varepsilon$. This is exactly what we wanted.

We have shown that $G=\langle\varepsilon\rangle$. In fact we can do more. Clearly $G$ cannot be finite. Because otherwise $\varepsilon$ must have finite order, i.e. $k\varepsilon=0$ for some positive integer $k$. Then $0\le\varepsilon\le 2\varepsilon\le\dots\le k\varepsilon=0$ by Lemma 2. So all of these must be equalities (by antisymmetry), i.e. $\varepsilon=0$, a contradiction.

So our restrictions have not only worked, we’ve shown that all groups with these properties essentially have the same structure, that of the infinite cyclic group. Let’s give $G$ a name: we say that the group $G$ is well-ordered if the set $G^+$ is well-ordered under $\le$. We have thus proved:

Proposition 1. The only non-trivial well-ordered group is the group $(\mathbb Z,+)$ of integers (up to isomorphism).

Now we can give one-line proofs of the following facts using our Proposition 1: (here any ordering is under the usual $\le$ order in $\mathbb R$)

Corollary 1. The $\gcd$ of two natural numbers exists, and is their least positive linear combination.

Proof. For $a,b\in\mathbb N$, the additive group $G=\{ax+by:x,y\in\mathbb Z\}$ is well-ordered, and so is equal to $\langle\varepsilon\rangle$ for $\varepsilon$ the least positive element of $G$. $\square$

Corollary 2. $\mathbb Z$ is a PID.

Proof. Any ideal in $\mathbb Z$ is a well-ordered group, and so must be $\langle d\rangle$ for some $d$. $\square$

Corollary 3. If $x_0+y_0\sqrt d$ is the least solution $>1$ to Pell’s equation $x^2-dy^2=1$, then all solutions are given by $x_n+y_n\sqrt d=(x_0+y_0\sqrt d)^n$ for $n\in\mathbb Z$.

Proof. The solutions $x_n+y_n\sqrt d$ to Pell’s equation form a subgroup of the multiplicative group of units in the ring $\mathbb Z[\sqrt d]$. Since it is well-ordered, the conclusion follows. $\square$

We can even improve Proposition 0 a little:

Corollary 4. If $S$ is a subgroup of $(\mathbb R,+)$, then the following are equivalent:

(i) $S$ is well-ordered;

(ii) $S$ is not dense;

(iii) $S$ is cyclic.

Proof. $(i)\Leftrightarrow (iii)$ by Proposition 1. $(iii)\Rightarrow (ii)$ is clear. Suppose that $S$ is not dense. Then there exist $a,b\in S$ with $a such that $(a,b)\cap S=\emptyset$. Then $(0,b-a)\cap S=\emptyset$, because $x\in (0,b-a)\cap S\Rightarrow a+x\in (a,b)\cap S$. Therefore $b-a$ is the minimal element in $S^+\backslash\{0\}$, i.e. $S=\langle b-a\rangle$. Thus $(ii)\Rightarrow (iii)$. $\square$

A consequence of Corollary 4 is:

Corollary 5. Let $G$ be a group. If $G$ has a faithful one-dimensional real representation $\rho: G\to\mathbb R^\times$, then $\rho(G)$ is dense if and only if $G\not\cong\mathbb Z/\mathbb Z,\mathbb Z/2\mathbb Z, \mathbb Z$.

Note that $\rho(G)$ is dense if and only if it has two $\mathbb Z$linearly independent elements; therefore $G$ must be torsion-free. And since $\rho$ is faithful, $G$ must be abelian: $\rho(ghg^{-1}h^{-1})=1\Rightarrow ghg^{-1}h^{-1}=e$, i.e. $gh=hg$ for all $g,h\in G$. Thus we get the following nice result:

Proposition 2. If $G$ has a faithful one-dimensional real representation, then one of the following holds:

(i) $G\cong\mathbb Z/\mathbb Z$;

(ii) $G\cong\mathbb Z/2\mathbb Z$;

(iii) $G\cong\mathbb Z$;

(iv) $G\triangleright\mathbb Z\oplus\mathbb Z$.

Moreover, if $|G|>2$ and $G$ is finitely generated, then $G\cong\mathbb Z^n$, for some $n$.