# Singular Conics and Intersection of Line with a Conic

I've been working through Silverman and Tate's book Rational Points on Elliptic Curves. They use conic equations as an introduction to singular/nonsingular curves. I've reproduced the problem with my comments below, then my questions follow.

Let $C$ be the conic given by the equation $$F(x,y) = ax^2 + bxy + cy^2 + dx + ey + f = 0$$ and let $\delta$ be the determinant $$\det \begin{bmatrix} 2a & b & d \\ b & 2c & e \\ d & e & 2f \\ \end{bmatrix}$$

(a) Show that if $\delta \neq 0$ then $C$ has no singular points.

(b) Conversely, show that if $\delta = 0$ and $b^2 - 4ac \neq 0$ then there is a unique singular point on $C$.

Given these two conditions we can classify the curve as two intersecting lines or a single point. So there is one singular point on the curve. This is from wikipedia.

(c) Let $L$ be the line $y = \alpha x + \beta$ with $\alpha \neq 0$. Show that the intersection of $L$ and $C$ consists of either zero, one, or two points.

Substitute the equation of the line into the equation of the conic. Then we get a quadratic equation $(a + \alpha b + \alpha^2 c)x^2 + (\beta b + 2\alpha\beta c + d + \alpha e)x + (c\beta^2 + \beta e + f) = 0$. This equation has zero, one, or two solutions.

(d) Determine the conditions on the coefficients which ensure that the intersection $L \cap C$ consists of exactly one point. What is the geometric significance of these conditions?

Let $(a + \alpha b + \alpha^2 c)x^2 + (\beta b + 2\alpha\beta c + d + \alpha e)x + (c\beta^2 + \beta e + f) = a'x^2 + b'x + c'$ There is exactly one solution if the determinant $b'^2 - 4a'c'$ of the quadratic equation is 0. Geometrically, the intersections collide: the line must be tangent.

My questions:

(b) Can anyone provide a reference other than Wikipedia for this? I've searched a lot but can't find anything that actually gives a derivation.

(c,d) What is the condition for zero, one, two roots? The field is not specified so I assume these roots could be complex. Then

• we have zero roots if $a' = b' = 0$
• we have one root if $a' = 0, b' \neq 0$
• we have two roots if $a' \neq, b' \neq 0$

But the two roots could be one root of multiplicity two. Is that correct? Or should I only be thinking about the third case (over real numbers), then considering the possible roots, with complex roots left out.

(d) How do I flesh out a proof of this fact? (i.e. How would I prove that $b'^2 - 4a'c'$ implies the line is tangent?)

• The line is tangent to a curve if infinitesimal order of distance between them near an intersection point is more than infinitesimal order of distance to that point – Alexey Burdin Apr 26 '15 at 0:40
• (a) is not answered as you say; for, Silverman does not expect an answer using "projective conic": it is an exercise 1.2 in the book; no tools have been mentioned, let alone developed. How did you answer (a)---I should like to know---in a more elementary way? – Wulfgang Mar 5 '17 at 5:09
• You're right that the other answer I linked to is quite a bit more complex than is necessary. I ended up answering: $\delta \neq 0$ implies that the matrix is invertible, i.e. that the only solution to $Ax = 0$ is $x = 0$. But there is no point $(0,0,0)$ in projective space. So $C$ has no singular points. – positivepeter Mar 6 '17 at 22:04

b):

Let $$Q(x,y) = ax^2 + 2hxy + by^2 + 2gx + 2fy + c$$

$$Q_x(x,y) = 2ax + 2hy + 2g$$ and $$Q_y(x,y) = 2hx + 2by+2f$$

Solving $$Q_x = 0$$ and $$Q_y = 0$$, $$\begin{bmatrix}a &h\\ h & b \end{bmatrix}\begin{bmatrix}x\\ y \end{bmatrix} = \begin{bmatrix}-g\\ -f \end{bmatrix}$$

Given $$ab - h^2 \ne 0$$, there is a unique solution for $$Q_x = 0, Q_y = 0$$ and $$Q$$ has a unique centre. Let that centre be $$(u ,v)$$. Shifting the centre of $$Q$$ to origin with $$x = X + u$$ and $$y = Y + v$$ gives $$R(X, Y) = aX^2 + 2hXY + bY^2 + \dfrac{\Delta}{\gamma}$$

where $$\Delta$$ is determinant of matrix of $$Q$$ and $$\gamma = ab - h^2$$. Given $$\Delta = 0$$, origin lies on $$R(X,Y)$$. $$0 = R(0,0) = Q(u, v)$$

Therefore, there exist only one point $$(u, v)$$ for $$Q_x = 0, Q_y = 0$$ and $$Q = 0$$. Hence $$(u, v)$$ is unique singular point of $$Q$$.

c):

Let $$L(t) = (u + tX, b + tY)$$, putting this in $$Q = 0$$ give $$pt^2 + qt + r = 0$$ where

$$\begin{cases}p = aX^2 + 2hXY + bY^2 \\ q = X Q_x(u, v) + Y Q_y (u, v) \\ r = Q(u,v) \end{cases}$$

Hence $$L$$ intersects $$Q$$ at one point, two points or never provided $$p, q, r$$ all are not zero.

d): I don't understand this part. $$L$$ is tangent to $$Q$$ when $$q^2 - 4pr = 0$$ but finding the condition on coefficients of $$Q$$ and $$L$$ is a lot of dull calculations (at least with my method).