Polar curve

In algebraic geometry, the first polar, or simply polar of an algebraic plane curve C of degree n with respect to a point Q is an algebraic curve of degree n−1 which contains every point of C whose tangent line passes through Q. It is used to investigate the relationship between the curve and its dual, for example in the derivation of the Plücker formulas.

1 Definition
2 Class of a curve
3 Higher polars
4 Poles
5 The Hessian
6 See also
7 References

Definition

Let C be defined in homogeneous coordinates by f(x, y, z) = 0 where f is a homogeneous polynomial of degree n, and let the homogeneous coordinates of Q be (a, b, c). Define the operator

$\Delta_Q = a{\partial\over\partial x}%2Bb{\partial\over\partial y}%2Bc{\partial\over\partial z}.$

Then Δ_Qf is a homogeneous polynomial of degree n−1 and Δ_Qf(x, y, z) = 0 defines a curve of degree n−1 called the first polar of C with respect of Q.

If P=(p, q, r) is a non-singular point on the curve C then the equation of the tangent at P is

$x{\partial f\over\partial x}(p, q, r)%2By{\partial f\over\partial y}(p, q, r)%2Bz{\partial f\over\partial z}(p, q, r)=0.$

In particular, if P is on the intersection of C and its first polar with respect to Q if and only if Q is on the tangent to C at P. Note also that for a double point of C, the partial derivatives of f are all 0 so the first polar contains these points as well.

Class of a curve

The class of C may be defined as the number of tangents that may be drawn to C from a point not on C (counting multiplicities and including imaginary tangents). Each of these tangents touches C at one of the points of intersection of C and the first polar, and by Bézout's theorem theorem there are at most n(n−1) of these. This puts an upper bound of n(n−1) on the class of a curve of degree n. The class may be computed exactly by counting the number and type of singular points on C (see Plücker formula).

Higher polars

The p-th polar of a C for an natural number p is defined as Δ_Q^pf(x, y, z) = 0. This is a curve of degree n−p. When p is n−1 the p-th polar is a line called the polar line of C with respect to Q. Similarly, when p is n−2 the curve is called the polar conic of C.

Using Taylor series in several variables and exploiting homogeneity, f(λa+μp, λb+μq, λc+μr) can be expanded in two ways as

$\mu^nf(p, q, r) %2B \lambda\mu^{n-1}\Delta_Q f(p, q, r) %2B \frac{1}{2}\lambda^2\mu^{n-2}\Delta_Q^2 f(p, q, r)%2B\dots$

and

$\lambda^nf(a, b, c) %2B \mu\lambda^{n-1}\Delta_P f(a, b, c) %2B \frac{1}{2}\mu^2\lambda^{n-2}\Delta_P^2 f(a, b, c)%2B\dots .$

Comparing coefficients of λ^pμ^n−p shows that

$\frac{1}{p!}\Delta_Q^p f(p, q, r)=\frac{1}{(n-p)!}\Delta_P^{n-p} f(a, b, c).$

In particular, the p-th polar of C with respect to Q is the locus of points P so that the (n−p)-th polar of C with respect to P passes through Q.^[1]

Poles

If the polar line of C with respect to a point Q is a line L, the Q is said to be a pole of L. A given line has (n−1)² poles (counting multiplicities etc.) where n is the degree of C. So see this, pick two points P and Q on L. The locus of points whose polar lines pass through P is the first polar of P and this is a curve of degree n−1. Similarly, the locus of points whose polar lines pass through Q is the first polar of Q and this is also a curve of degree n−1. The polar line of a point is L if and only if it contains both P and Q, so the poles of L are exactly the points of intersection of the two first polars. By Bézout's theorem these curves have (n−1)² points of intersection and these are the pole of L.^[2]

The Hessian

For a given point Q=(a, b, c), the polar conic is the locus of points P so that Q is on the second polar of P. In other words the equation of the polar conic is

$\Delta_{(x, y, z)}^2 f(a, b, c)=x^2{\partial^2 f\over\partial x^2}(a, b, c)%2B2xy{\partial^2 f\over\partial x\partial y}(a, b, c)%2B\dots=0.$

The conic is degenerate if and only if the determinant of the Hessian of f,

$H(f) = \begin{bmatrix} \frac{\partial^2 f}{\partial x^2} & \frac{\partial^2 f}{\partial x\,\partial y} & \frac{\partial^2 f}{\partial x\,\partial z} \\ \\ \frac{\partial^2 f}{\partial y\,\partial x} & \frac{\partial^2 f}{\partial y^2} & \frac{\partial^2 f}{\partial y\,\partial z} \\ \\ \frac{\partial^2 f}{\partial z\,\partial x} & \frac{\partial^2 f}{\partial z\,\partial y} & \frac{\partial^2 f}{\partial z^2} \end{bmatrix}.$

Therefor the equation |H(f)|=0 defines a curve, the locus of points whose polar conics are degenerate, of degree 3(n−2) called the Hessian curve of C.

References

^ Follows Salmon pp. 49-50 but essentially the same argument with different notation is given in Basset pp. 16-17.
^ Basset p. 20, Salmon p. 51

Basset, Alfred Barnard (1901). An Elementary Treatise on Cubic and Quartic Curves. Deighton Bell & Co.. pp. 16ff.. http://books.google.com/books?id=yUxtAAAAMAAJ.
Salmon, George (1879). Higher Plane Curves. Hodges, Foster, and Figgis. pp. 49ff.. http://www.archive.org/details/treatiseonhigher00salmuoft.
Section 1.2 of Fulton, Introduction to intersection theory in algebraic geometry, CBMS, AMS, 1984.
Ivanov, A.B. (2001), "Polar", in Hazewinkel, Michiel, Encyclopedia of Mathematics, Springer, ISBN 978-1556080104, http://www.encyclopediaofmath.org/index.php?title=P/p073400
Ivanov, A.B. (2001), "Hessian (algebraic curve)", in Hazewinkel, Michiel, Encyclopedia of Mathematics, Springer, ISBN 978-1556080104, http://www.encyclopediaofmath.org/index.php?title=H/h047150