How to tell if a second-order curve goes below the $x$ axis? - MathOverflow

How to tell if a second-order curve goes below the $x$ axis?

2013-02-11T15:06:42Z

Suppose we have a second-order curve in general form:

(1) $a_{11}x^{2}+2a_{12}xy+a_{22}y^{2}+2a_{13}x+2a_{23}y+a_{33}=0$.

I'd like to know if there is a simple condition that ensures that the curve has at least one point on on or below the $x$ axis, i.e. that the left-hand side of (1) is nonpositive.

In the trivial case that the curve is a parabola, the discriminant being nonnegative is just such a condition. But what happens in the general case?

Answer by Peter Michor for How to tell if a second-order curve goes below the $x$ axis?

2013-02-11T15:54:11Z

Solve for $y$ in the form $y= A(x) \pm \sqrt{B(x)}$ and estimate. More abstract versions are just variant of this.

Answer by Serge Lvovski for How to tell if a second-order curve goes below the $x$ axis?

2013-02-11T18:15:33Z

We may regard the left-hand side of the equation of thecurve as a quadratic polynomial in $x$. If $D(y)$ is its discriminant (with respect to $x$), then $D(y)\ge 0$ iff there exists a point with the second coordinate $y$ on the curve. Solve this inequality for $y$ and check whether its minimal solution is negative:)))