# The Stacks Project

## Tag 0AAP

Exercise 102.54.3. Let $X = \mathbf{A}^2_\mathbf{C}$ where $\mathbf{C}$ is the field of complex numbers. A line will mean a closed subscheme of $X$ defined by one linear equation $ax + by + c = 0$ for some $a, b, c \in \mathbf{C}$ with $(a, b) \not = (0, 0)$. A curve will mean an irreducible (so nonempty) closed subscheme $C \subset X$ of dimension $1$. A quadric will mean a curve defined by one quadratic equation $ax^2 + bxy + cy^2 + dx + ey + f = 0$ for some $a, b, c, d, e, f \in \mathbf{C}$ and $(a, b, c) \not = (0, 0, 0)$.

1. Find a curve $C$ such that every line has nonempty intersection with $C$.
2. Find a curve $C$ such that every line and every quadric has nonempty intersection with $C$.
3. Show that for every curve $C$ there exists another curve such that $C \cap C' = \emptyset$.

The code snippet corresponding to this tag is a part of the file exercises.tex and is located in lines 5625–5644 (see updates for more information).

\begin{exercise}
\label{exercise-miss-curve}
Let $X = \mathbf{A}^2_\mathbf{C}$ where $\mathbf{C}$ is the field
of complex numbers. A {\it line} will mean a closed
subscheme of $X$ defined by one linear equation $ax + by + c = 0$ for
some $a, b, c \in \mathbf{C}$ with $(a, b) \not = (0, 0)$.
A {\it curve} will mean an irreducible (so nonempty) closed subscheme
$C \subset X$ of dimension $1$.
A {\it quadric} will mean a curve defined by one
quadratic equation $ax^2 + bxy + cy^2 + dx + ey + f = 0$
for some $a, b, c, d, e, f \in \mathbf{C}$ and
$(a, b, c) \not = (0, 0, 0)$.
\begin{enumerate}
\item Find a curve $C$ such that every line has nonempty intersection with $C$.
\item Find a curve $C$ such that every line and every quadric has nonempty
intersection with $C$.
\item Show that for every curve $C$ there exists another curve
such that $C \cap C' = \emptyset$.
\end{enumerate}
\end{exercise}

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