path: root/controle-20260415.tex

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\begin{document}
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\title{FMA-4AC05-TP / ACCQ205\\Final exam — answer key\\{\normalsize Algebraic curves}}
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\title{FMA-4AC05-TP / ACCQ205\\Final exam\\{\normalsize Algebraic curves}}
\fi
\author{}
\date{2026-04-15}
\maketitle

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\noindent\textbf{Instructions.}

This exam consists of \textcolor{red}{XXX} completely independent exercises.  They
can be tackled in any order, but students must clearly and readably
indicate where each exercise starts and ends.

\medbreak

Answers can be written in English or French.

\medbreak

Use of written documents of any kind (such as handwritten or printed
notes, exercise sheets or books) is permitted.

Use of electronic devices of any kind is prohibited.

\medbreak

Duration: 2 hours

\ifcorrige
This answer key has \textcolor{red}{XXX} pages (this cover page included).
\else
This exam has \textcolor{red}{XXX} pages (this cover page included).
\fi

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\exercise

We say that a set of seven distinct points $p_1,\ldots,p_7$ in the
projective plane $\mathbb{P}^2$ over a field $k$ is a \textbf{Fano
  configuration} when the points satisfy the alignment conditions
depicted in the following figure:

\begin{center}
\vskip-2ex\leavevmode
\begin{tikzpicture}
\coordinate (P1) at (-2cm,0);
\coordinate (P2) at (2cm,0);
\coordinate (P3) at (0,0);
\coordinate (P4) at (0,3.464cm);
\coordinate (P5) at (-1cm,1.732cm);
\coordinate (P6) at (1cm,1.732cm);
\coordinate (P7) at (0cm,1.155cm);
\draw (P2)--(P4);
\draw (P1)--(P4);
\draw (P3)--(P4);
\draw (P1)--(P2);
\draw (P2)--(P5);
\draw (P1)--(P6);
\draw (P3) to[out=180,in=240] (P5) to[out=60,in=120] (P6) to[out=300,in=0] (P3);
\fill[black] (P1) circle (2.5pt);
\fill[black] (P2) circle (2.5pt);
\fill[black] (P3) circle (2.5pt);
\fill[black] (P4) circle (2.5pt);
\fill[black] (P5) circle (2.5pt);
\fill[black] (P6) circle (2.5pt);
\fill[black] (P7) circle (2.5pt);
\node[anchor=north east] at (P1) {$p_1$};
\node[anchor=north west] at (P2) {$p_2$};
\node[anchor=north] at (P3) {$p_3$};
\node[anchor=south] at (P4) {$p_4$};
\node[anchor=south east] at (P5) {$p_5$};
\node[anchor=south west] at (P6) {$p_6$};
\node[anchor=north west] at (P7) {$p_7$};
\end{tikzpicture}
\vskip-5ex\leavevmode
\end{center}

This means: the seven points are distinct; all the following sets of
points are aligned: $\{p_2, p_4, p_6\}$, $\{p_1, p_4, p_5\}$, $\{p_3,
p_4, p_7\}$, $\{p_1, p_2, p_3\}$, $\{p_2, p_5, p_7\}$, $\{p_1, p_6,
p_7\}$ and $\{p_3, p_5, p_6\}$; and no other set of three of the $p_i$
are aligned.

The goal of this exercise is to determine over which fields $k$ a Fano
configuration exists, and compute the coordinates of its points.

We fix an arbitrary field $k$.  The word “point”, in what follows,
will refer to an element of $\mathbb{P}^2(k)$, in other words, a point
with coordinates in $k$ (that is, a $k$-point).

We shall denote by $(x{:}y{:}z)$ the (homogeneous) coordinates of a
point, and write $[u{:}v{:}w]$ for the line $\{ux+vy+wz = 0\}$.
%% Recall that the line through $(x_1{:}y_1{:}z_1)$ and
%% $(x_2{:}y_2{:}z_2)$ (assumed distinct) is given by the formula $[(y_1
%%   z_2 - y_2 z_1) : (z_1 x_2 - z_2 x_1) : (x_1 y_2 - x_2 y_1)]$, and
%% that the same formula (exchanging parentheses and square brackets) can
%% also be used to compute the intersection of two distinct lines.  (This
%% may not always be the best or simplest way to compute coordinates,
%% however!)

\emph{We assume for questions (1)–(3) below that $p_1,\ldots,p_7$ is a
Fano configuration of points (over the given field $k$), and the
questions will serve to compute the coordinates of the points.}  We
denote $\ell_{ijk}$ the line through $p_i,p_j,p_k$ when it exists.

\textbf{(1)} Explain why we can assume, without loss of generality,
that $p_4=(1{:}0{:}0)$ and $p_2=(0{:}1{:}0)$ and $p_1=(0{:}0{:}1)$ and
$p_7=(1{:}1{:}1)$.  \emph{We shall now do so.}

\begin{answer}
No three of the four points $p_4,p_2,p_1,p_7$ are aligned, so they are
a projective basis of $\mathbb{P}^2$: thus, there is a unique
projective transformation of $\mathbb{P}^2$ mapping them to the
standard basis $(1{:}0{:}0), \penalty-100 (0{:}1{:}0), \penalty-100
(0{:}0{:}1), \penalty-100 (1{:}1{:}1)$.  Since projective
transformations preserve alignment, we can apply this projective
transformation and assume that $p_4=(1{:}0{:}0)$ and $p_2=(0{:}1{:}0)$
and $p_1=(0{:}0{:}1)$ and $p_7=(1{:}1{:}1)$.
\end{answer}

\textbf{(2)} Compute the coordinates (i.e., equations) of the lines
$\ell_{123}$ and $\ell_{347}$, and deduce the coordinates of the point
$p_3$.  Analogously compute the coordiantes of $p_5$ and $p_6$.

\begin{answer}
Denoting $p\vee q$ the line through distinct points $p$ and $q$, we
get $\ell_{123} = p_1 \vee p_2 = [1{:}0{:}0]$ and $\ell_{347} =
p_4\vee p_7 = [0{:}-1{:}1]$.  Denoting by $\ell\wedge m$ the point of
intersection of distinct lines $\ell$ and $m$, we get $p_3 =
\ell_{123} \wedge \ell_{347} = (0{:}1{:}1)$.

Similar computations (or using the cyclic symmetry $p_1 \mapsto p_2
\mapsto p_4 \mapsto p_1$ which corresponds to a cyclic permutation of
coordinates) gives $p_5 = (1{:}0{:}1)$ and $p_6 = (1{:}1{:}0)$.
\end{answer}

\textbf{(3)} Using the last alignment condition that hasn't yet been
used, give a \emph{necessary} condition for a Fano configuration to
exist in $\mathbb{P}^2(k)$.

\begin{answer}
The remaining condition is the alignment of $p_3,p_5,p_6$.  This is
expressed by the vanishing of the determinant of their coordinates,
or, equivalently, by computing $p_3 \vee p_5 = [1{:}1{:}-1]$ and
expressing the fact that $p_6$ lies on it.  The necessary condition we
get is: $2=0$ in $k$, in other words, the field $k$ is of
characteristic $2$.

Thus, we have shown that a Fano configuration does not exist in a
field of characteristic $\neq 2$.
\end{answer}

\textbf{(4)} Conversely, use the previously computed coordinates to
explain why this necessary condition on $k$ is also sufficient for a
Fano configuration to exist.

\begin{answer}
If $k$ is any field, then setting $p_4=(1{:}0{:}0)$ and
$p_2=(0{:}1{:}0)$ and $p_1=(0{:}0{:}1)$ and $p_7=(1{:}1{:}1)$ and $p_3
= (0{:}1{:}1)$ and $p_5 = (1{:}0{:}1)$ and $p_6 = (1{:}1{:}0)$ ensures
six of the seven required alignments.  And if $k$ is of
characteristic $2$ then $p_3,p_5,p_6$ are also aligned for the reasons
explained in the previous question.  But furthermore, this gives an
identification of the $7$ points with the points of
$\mathbb{P}^2(\mathbb{F}_2)$ (where $\mathbb{F}_2$ is seen as a
subfield of $k$), and since $\mathbb{P}^2(\mathbb{F}_2)$ has
$7$ lines, there are no other alignments than the prescribed ones.
\end{answer}

\medskip

\textbf{(5)} \emph{Independently of all previous questions,} show that
the number of labeled projective bases in $\mathbb{P}^2(\mathbb{F}_q)$
(in other words, $4$-uples $(a,b,c,d)$ of points such that no $3$ are
aligned) is given by the formula: $q^3\, (q-1)^2\, (q+1)\, (q^2+q+1)$.

\emph{Hint:} One possible approach is to count the number of possible
choices for point $a$, then $b$, then $c$, then $d$; another possible
approach is to count matrices in $\mathit{GL}_3(\mathbb{F}_q)$ by
counting the possibilities for the first, then second, then third
columns, and deduce the cardinality of $\mathit{PGL}_3(\mathbb{F}_q)$.
Both approaches give the same formula (although in a slightly
different way).

\begin{answer}
First aproach: there are $q^2+q+1$ possibilities for the point $a$,
because that is the cardinality of $\mathbb{P}^2(\mathbb{F}_q)$.  For
the point $b$, since it needs to be different from $a$, we are left
with $q^2+q$ possibilities.  For the point $c$, since it cannot belong
to the line $ab$, which has $q+1$ points, we are left with $q^2$
possibilities.  Finally, for the last point $d$, there are three lines
to be ruled out ($ab$, $ac$ and $bc$), each one having $q+1$ points,
but as they meet pairwise in a single point, they have $3(q+1)-3 = 3q$
point together, and we are left with $(q^2+q+1)-3q = q^2-2q+1 =
(q-1)^2$ possibilities for $d$.  This means there are $(q^2+q+1)\,
(q^2+q)\, q^2\, (q-1)^2 = q^3\, (q-1)^2\, (q+1)\, (q^2+q+1)$ labeled
projective bases.

Second approach: to construct a matrix in
$\mathit{GL}_3(\mathbb{F}_q)$, we first choose its first column, which
can be any nonzero vector, giving us $q^3-1$ possibilities; then we
choose its second, which can be any vector not collinear with the
first, giving us $q^3-q$ possibilities; then we choose the third,
which can be any vector not in the vector space spanned by the first
two, leaving us $q^3-q^2$ possibilities.  Thus, there are
$(q^3-1)\,(q^3-q)\,(q^3-q^2)$ elements in
$\mathit{GL}_3(\mathbb{F}_q)$.  But since
$\mathit{PGL}_3(\mathbb{F}_q)$ is its quotient by the subgroup of
order $q-1$ consisting of homotheties (multiplication by a nonzero
constant), there are $\frac{(q^3-1)\,(q^3-q)\,(q^3-q^2)}{q-1} =
(q^2+q+1)\,(q^3-q)\,(q^3-q^2) = q^3\, (q-1)^2\, (q+1)\, (q^2+q+1)$
elements of $\mathit{PGL}_3(\mathbb{F}_q)$.  This is also the number
of labeled projective bases because $\mathit{PGL}_3(\mathbb{F}_q)$
acts simply transitively on such.
\end{answer}

\medskip

Let us now say that a \textbf{labeled Fano configuration}\footnote{In
French: “configuration de Fano étiquetée”.} is a $7$-tuple of points
$(p_1,\ldots,p_7)$ satisfying the same conditions as previously.  (In
other words, the difference is that the unlabeled Fano configuration
is just the set $\{p_1,\ldots,p_7\}$ of seven points, whereas the
labeled Fano configuration has the information of which is $p_1$,
which is $p_2$, etc.)

\smallskip

\textbf{(6)} How many labeled Fano configurations are there in
$\mathbb{P}^2(\mathbb{F}_{2^d})$?  Compute this number of $d=1$ and
$d=2$ (that is, in $\mathbb{P}^2(\mathbb{F}_2)$ and
$\mathbb{P}^2(\mathbb{F}_4)$).

\emph{Note:} You can write numbers as products, there is no need to
fully compute the multiplications by hand.

\begin{answer}
We have seen in questions (1)–(4) that, over a field of
characteristic $2$, a labeled Fano configuration is constructed in a
unique way from a labeled projective basis (which serves as
$p_4,p_2,p_1,p_7$).  Thus, the number of Fano configurations in
$\mathbb{P}^2(\mathbb{F}_{2^d})$ equals the number of labeled
projective bases, which equals $2^{3d} (2^d-1)^2 (2^d+1)
(2^{2d}+2^d+1)$.  For $d=1$ this gives $8\times 3\times 7 = 168$; and
for $d=2$ this gives $64\times 9 \times 5 \times 21 = 60\,480$.
\end{answer}

\textbf{(7)} Deduce the number of \emph{unlabeled} Fano configurations
in $\mathbb{P}^2(\mathbb{F}_4)$.  (\emph{Hint:} The previous question
provides a way to count the number of labeled Fano configurations for
each unlabeled one!)

\begin{answer}
The number of ways to label a given Fano configuration, i.e., the
number of labeled Fano configurations for each unlabeled one, equals
the number of Fano configurations in $\mathbb{P}^2(\mathbb{F}_2)$,
which we have seen is $168$.  So we are left with $60\,480 \, / \, 168
= 360$ (unlabeled) Fano configurations in $\mathbb{P}^2(\mathbb{F}_4)$.
\end{answer}

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