\section{Quadratic fields}
We now specialise the results of the previous section to the case of
extensions of $\bbQ$ of degree 2. Such a field is of the form
$\bbQ[T]/P(T)$ where $P(T)$ is an irreducible polynomial of degree 2.
An order in such a field is generated by 1 and a non-rational element
$\alpha$ that satisfies an equation of the type $P(T)=T^2-bT+c$. Thus
every order has the form $R[T]/P(T)$. Now, it is clear that
$\Trace(\alpha)=b$ and $\Nm(\alpha)=c$. Moreover,
$\Trace(\alpha^2)=\Trace(b\alpha-c)=b^2-2c$. Thus the discriminant
$D_R$ of $R$ is the determinant of
%
\( \left(
\begin{smallmatrix} 2 & b\\
b & b^2-2c\end{smallmatrix}
\right)
\)
%
which is $b^2-4c$ (as expected). In particular, we see that
$D_R=b^2\pmod{4}$; \ie the discriminant must be 0 or 1 modulo 4. In
the first case, we can replace $\alpha$ by $\alpha+(b-1)/2$ so that we
get an element with trace 1. In the second case, we can replace
$\alpha$ by $\alpha+b/2$ to get an element with trace 0. Thus we can
assume that the equation takes the form $T^2-T+N$ in the first case
and $T^2+N$ in the second case. An alternative normalisation is to
replace $\alpha$ by $\omega_D=(D_R+\sqrt{D_R})/2$ in both cases; this
can be done since $D_R+b$ is even in both cases. We thus have a
natural basis for $R$. There is also a natural involution on $R$ which
sends $\sqrt{D_R}$ to $-\sqrt{D_R}$ or equivalently $\omega_D$ to
$D_R-\omega_D$.
\subsection{Prime ideals}
By the earlier analysis, we see that every prime ideal is either
ramified (to order 2) or of degree 1 or of degree 2. If the prime lies
over $2$ then it is not ramified when $D_R$ is odd since, in that case
the equation takes the form $T^2+T$ or $T^2+T+1$ modulo 2; both these
equations have distinct roots. When $D_R$ is even, the prime over $2$
is ramified. When the prime lies over an odd prime $p$, it is clear
that the prime is ramified when the discriminant is divisible by
$p$. Thus the ramified primes are precisely those that lie over primes
$p$ that divide the discriminant. (This is also true for {\em any}
field extension of $\bbQ$ that is normal in the sense that it contains
all the roots of the polynomial that defines it).
If $D_R$ is odd and in the above notation $N$ is even, then the primes
lying over 2 and $\bbZ\cdot 2+\bbZ\cdot\alpha$ and $\bbZ\cdot
2+\bbZ\cdot(1-\alpha)$. When $N$ is odd, then the only prime lying
over 2 is $2R$. Now, if $p$ is an odd prime that does not divide the
discriminant then either $\bbF_p[T]/(P(T))$ is isomorphic to
$\bbF_{p^2}$ or it splits into two $\bbF_p$ factors. The former case
occurs when $D_R$ is not a square modulo $p$ and in this case the
prime lying over $p$ is just the ideal $pR$; which is principal. In
the second case $D_R$ is a square modulo $p$ and we obtain two primes
$P_p$ and $Q_p$, lying over $p$; both these have norm $p$ and their
product (and intersection) is $pR$. Let $c_p$ be a number between $1$
and $p-1$ so that $c_p^2=D_R\pmod{p}$; then $a_p=(1+c_p)/2$ satisfies
the equation modulo $p$ in the $D_R$ odd case and $a_p=(c_p)/2$
satisfies the equation modulo $p$ in the $D_R$ even case. Thus we can
pick a solution $a_p$ of the equation modulo $p$ in each case and
declare that $P_p=\bbZ\cdot p+\bbZ\cdot(\alpha-a_p)$. The primes $P_p$
and $Q_p$ are interchanged by the involution.
\subsection{Naive computation of the class group}
As shown earlier, each element of the class group of the order $R$ is
represented by an invertible ideal $J$ with $\Nm(J)\leq\delta_R$; here
$\delta_R=\sqrt{|D_R|}$ if $D_R>0$ and $\delta_R=(2/pi)\sqrt{|D_R|}$
if $D_R<0$. Now, if $\overline{J}$ is the image of $J$ under the
involution, then we have seen above (by writing $J$ as a product of
primes) that $J\cdot\overline{J}=\Nm{J}R$. Thus the involution acts
on the class group by group inversion (which is a group homomorphism
for abelian groups!). In particular, we know how to represent inverses
in this set of representatives.
From the above discussion we see that one natural set of generators is
to pick one prime ideal $P_p$ lying over each split prime $p$ and for
each ramified prime $p$. We only need to consider primes satisfying
the criterion $p\leq\delta_R$; let $S$ denote the set of such primes.
Now we need to write relations. Suppose that $T$ is the (finite) set
of all integers $n$ such that (1) $n$ is a multiple of elements of $T$
(2) For each prime divisor $p$ of $n$, $n/p\leq\delta_R$. Each such
$n$ can be written uniquely as the norm of an ideal $J_n$ that is a
product of the ideals $P_p$. If we find an element $\alpha_n$ in $J_n$
so that $\Nm(\alpha_n)\leq n\cdot\delta_R\cdot$, then
$\alpha_n=J_n\cdot I_n$, where $\Nm(I_n)\leq\delta_R$. We can thus
write a natural factorisation of the ideal $\alpha_n$ in terms of
$P_p$ and $Q_p$. Note that when $n\geq\delta_R$, the existence of such
an $\alpha$ is guaranteed by the lemma proved in the previous section.
To write these relations, it is sufficient find all numbers less
than $\max(T)\delta_R$ which are products of primes in $S$ and write
these elements as norms.
Now suppose we have a relation $\prod_{p\in S}P_p^{n_p}=\alpha R$ with
$n_p\geq 0$. If $\Nm(\alpha)\geq\delta_R$, then we can find a factor
$\prod_{p\in S}P_p^{m_p}$ which has norm $n$ larger than $\delta_R$,
but lying in $T$. Then, we can replace the above relation by
\[ \overline{I_n}\cdot\prod_{p\in S}P_p^{n_p-m_p}=(\alpha/\alpha_n)\Nm(I_n)R \]
Now the left hand side has integral norm and so we have obtained
another relation. Moreover, the norm of the left hand side is smaller
than the earlier norm. Thus we can always reduce any relation to a
product of relations of the type given above.
To write the relations associated with elements of $T$ as above we
note that for each $n$ in $T$ we can construct a candidate for
$\alpha_n$ as follows. First of all we use Chinese Remainder theorem
to find an integer $a_n$ so that $a_n=a_p\pmod{p}$ for every $p$
dividing $n$ (if necessary we can actually use Hensel's lemma to
replace $a_p$ by the root of the equation modulo the maximal power of
$p$ that divides $n$). Then, elements of the form $x+y\alpha$ are
candidates where $y$ is some number less than $n$ and $x$ is the
reduction modulo $n$ of $a_n\cdot y$. In addition, we can impose the
condition that $x+y\alpha$ lies in a specified region in $\bbR\cdot K$
with volume $n\delta_R$ (this region is a rectangle in the case
$D_R>0$ and a circle in the case $D_R<0$). These conditions make the
search for $\alpha_n$ effective.
Now the numbers in $T$ could be just short of $\delta_R^2$, so that
the norm of $\alpha_n$ could be just short of $\delta_R^3$. This is in
general too big a collection of relations to handle. One way to
simplify the approach is to make reductions to the set $S$ on the
basis of relations found. Thus, if we find that $P_p$ has order $k$
based on relations already found then we do not consider numbers $n$
that are divisible by powers of $p$ larger than $k-1$. Similarly, if
we found a relation expression $P_p$ in terms of smaller primes in the
set $S$, then we can drop multiples of $p$ from further choices for
$n$ in $T$. Finally, we can use a ``Class Number formula'' to give an
{\em estimate} in terms of lower and upper bounds for the size of the
group. Once we find a group that is the correct range then there are
techniques to verify that there are no more relations to be
considered.
Thus the techniques described above {\em could} be used to compute the
class group even for large $D_R$. However, the main aim of this
section was to show the {\em possibility} of making the
computation. We will need some more effective techniques to deal with
finite abelian groups before we can make the computation more
efficient.
As a demonstration we now compute the class group of the discriminant
$257$. The associated polynomial is $T^2-T-64$. The initial
candidates for the set $S$ consist of the primes $\leq 16$, \ie the
set $\{2,3,5,7,11,13\}$. Now, the polynomial becomes $T^2+T$ modulo 2
so that $2$ is split so it is in $S$. Now we have
\begin{align*}
257 &\congruent 2 \pmod{3} &&\text{~and~} &257 &\congruent 2 \pmod{5}
\end{align*}
which shows that 3 and 5 are non-split and thus not in $S$. The
squares modulo 7 are 1, 4 and 2, while $257\congruent 5\pmod{7}$; thus
7 is not in $S$ either. We also check that
\begin{align*}
257 \congruent 4 &\congruent 2^2 \pmod{11} &&\text{~and~}
&257 \congruent 10 &\congruent 6^2\pmod{13}
\end{align*}
so that 11 and 13 are in $S$. We then see easily that $T$ is
\[
\{22=2\cdot 11, 26=2\cdot 13, 32=2^5, 121= 11^2, 143=11\cdot 13,
169=13^2\}
\]
We now compute the relations in succession. We lift the above roots
$0\pmod{2}$ and $7\pmod{11}$ (of the equation $T^2-T-64$) to the root
$18\congruent -4\pmod{22}$. Thus a candidate for $\alpha_{22}$ is
$\alpha+4$, which has norm $44=2^2\cdot 11$. Thus we obtain the
relation $P_2^2\cdot P_{11}$. Next we lift the roots $0\mod{2}$ and
$10\mod{13}$ to the root $10\mod{26}$. Thus a candidate for
$\alpha_{26}$ is $\alpha-10$ which has norm $26=2\cdot 13$. Thus we
obtain the relation $P_2\cdot P_{13}$. Next we have
$\alpha_{32}=\alpha$, which has norm $64=2^6$, which gives the
relation $P_2^6$. Next, we lift (using Hensel's lemma) the root
$7\pmod{11}$ to the root $18\pmod{121}$ which gives
$\alpha_{121}=\alpha-18$ which has norm $242=2\cdot 11^2$ so we have a
relation $P_2\cdot P_{11}^2$. Now, we could calculate further but we
notice that this says that the class group is a quotient of a group of
order $3$ that is generated by $P_2$. Since it is clear that this
ideal is {\em not} principal, it follows that the class group in this
case {\em is} $\bbZ/3\bbZ$. Note that we did not use the rectangular
bounds for the sizes of $\alpha_n$ in this computation since all the
numbers were ``small'' in any case, but in general we would need to
use these restrictions as well.
\subsection{Binary Quadratic Forms}
Gauss's approach to ideals (which were not defined in his time!) was
to represent elements of the class group (groups were also not defined
in his time!) by equivalence classes of quadratic forms. The idea is
to make use of the fact that for each ideal $I$ we are actually
interested in objects like $\Nm(\alpha)/\Nm(I)$ for some element
$\alpha$ in $I$. As seen above, the ideal class is represented by some
ideal $J$ with $\Nm(J)=\Nm(\alpha)/\Nm(I)$.
To fix notation, let the quadratic order $R$ be given as
$\bbZ+\bbZ\cdot\omega$, where $\omega=(D+\sqrt{D})/2$ with $D=D_R$ the
discriminant of the order $R$; then $\omega$ satisfies the equation
\[ \omega^2 - D\cdot \omega + \frac{D^2-D}{4} = 0 \]
Any non-zero ideal $I$ in $R$ is then of the form $\bbZ\cdot
a+\bbZ\cdot(b+c\omega)$, where $I\cap\bbZ=\bbZ\cdot a$ is the
restriction of $I$ to $\bbZ$; we can assume that $a>0$. Moreover, by
Euclidean division we can subtract a multiple of $a$ from $b$ to
ensure that $0\leq b0$. In the first case, we
restrict our attention to quadratic forms $Q(x,y)=qx^2+sxy-ry^2$
(continuing the above notation) such that $q>0$. Since $D=s^2+4qr<0$,
we see that $r<0$. In fact $Q(x,y)>0$ for all $(x,y)\neq(0,0)$.
Pictorially, the region $Q(x,y)\leq r$ is bounded by an ellipse. Thus,
among lattice points we can choose $u_1$ to be an element where the
$Q(u_1)$ takes its minimum (non-zero) value. Now, we can complete
$u_1$ to a basis by picking a suitable vector $u_2$. The only possible
alternative choices for $u_2$ are $nu_1\pm u_2$ for some integer $n$.
Let $u_2$ be so chosen that the value $Q(u_2)$ is minimum in this
collection. It is not too difficult to show that the expression for
$Q$ in this basis is independent of the finitely many choices
available. (In fact for $D|>4$ the choices of $u_1$ and $u_2$ are
unique upto sign). Now, in this basis we get $Q(x,y)=Ax^2+Bxy+Cy^2$
with $A\leq C$ and $|B|\leq A$. Moreover, if one of these is an
equality (which can only happen if $|D|\leq 4$), we have $B\geq 0$ as
well. A quadratic form with negative discriminant is said to be {\em
reduced} if it has this special form. Clearly, there are only
finitely many such forms for a given $D$; one for each equivalence
class of quadratic forms. Thus we have found representatives for the
class group.
When $D>0$, the quadratic forms are indefinite. The locus $Q(x,y)=r$
represents a hyperbola. Now the value 0 is not attained at non-zero
$(x,y)$ (else $D$ would have a square root in integers) and the values
are all integers. Thus, the absolute value of $Q$ attains a minimum at
some point $u_1$ on the lattice. But this $u_1$ is far from unique (in
fact there are infinitely many points where $Q$ takes this value. One
can show that upto a finite number of choices these are related by an
integer change of co-ordinates. Now, as before, $u_1$ can be completed
to a basis by a choice of $u_2$. The alternatives for this choice are
$nu_1\pm u_2$ as earlier. Again, there are only finitely many of these
with sign opposite to that of $Q(u_1)$ (since the term $n^2Q(u_1)$ in
the expansion of the quadratic form will dominate for $n$ large).
Among this finite set we choose $u_2$ so that the absolute value of
$Q$ is minimum (again with only finitely many options for this
choice). Thus, each equivalence class of quadratic forms has been
represented upto a finite ambiguity. Moreover, one can bound the
ambiguity depending on $D_R$.
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