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DUPIN INDICATRIX, ASYMPTOTIC DIRECTIONS AND LINES

Dupin indicatrix of a surface at a point. Consider an xy-coordinate system where
the x-axis represents the direction of minimum
curvature k_{1} and the y-axis represents the
direction of maximum curvature k_{2} at some point
P of a surface S. The Dupin indicatrix of the
surface at point P depends on the value of the
total (Gaussian) curvature K = k_{1}k_{2} at point P as
follows:

K > 0. The Dupin indicatrix is the ellipse

|k_{1}| x^{2} + |k_{2}| y^{2} = 1

See Fig. 1.

K < 0. The Dupin indicatrix is a set of conjugate hyperbolas

k_{1} x^{2} + k_{2} y^{2} = 1

k_{1} x^{2} + k_{2} y^{2} = -1

See Fig. 2.

K = 0. The Dupin indicatrix is a set of parallel lines corresponding to one of the forms

x^{2} = |1/k_{1}| or y^{2} = |1/k_{2}|

See Fig. 3.

We will now examine these three cases in detail.

Case 1. K > 0. k_{1} and k_{2} have the same sign,
the surface at point P is bowl shaped, and point P is an elliptic point. Let us suppose that k_{1} and
k_{2} are both positive. Then the Dupin indicatrix at point P is the ellipse

1) k_{1}x^{2} + k_{2}y^{2} = 1

shown in Fig. 1. Let us compute the distance r from the origin to point A. The equation of line OA is

2) y = x tan α .

We find the coordinates (x, y) of point A by solving equations 1) and 2) simultaneously. If we substitute 2) into 1) we obtain

The distance r is then given by

and since

k_{1}(α) = k_{1} cos^{2} α + k_{2} sin^{2} α ,

where k_{n}(α) is the normal curvature at point P in the direction α , we have

where ρ_{n}(α) is the radius of normal curvature in the direction α. The x and y intercepts Oa and
Ob are

where ρ_{1} and ρ_{2} are the radii of normal curvature corresponding to k_{1} and k_{2}.

Case 2. K < 0. k_{1} and k_{2} have opposite signs, the surface at point P is saddle shaped, and
point P is a hyperbolic point. The Dupin indicatrix is a set of conjugate hyperbolas

k_{1} x^{2} + k_{2} y^{2} = 1

k_{1} x^{2} + k_{2} y^{2} = -1

shown in Fig. 2. As in Case 1, and by the same type analysis, the values of r, Oa and Ob are

The two asymptotes of the hyperbolas are lines representing directions at which k_{n}(α) = 0. As the
angle α increases and passes over an asymptote a passage occurs from one hyperbola to the other
along with a change in sign of k_{n}(α).

Case 3. K = 0. Either k_{1} = 0 or k_{2} = 0, the surface at point P is a parabolic cylinder, and
point P is a parabolic point. If k_{1} = 0 the Dupin indicatrix is a set of parallel lines corresponding
to the equation

y^{2} = |1/k_{2}| .

If k_{2} = 0 the Dupin indicatrix is a set of parallel lines corresponding to the equation

x^{2} = |1/k_{1}| .

As in the other cases, and by the same type analysis, the values of r, and Oa are

Overview. The above three cases represent the different locii to which the equation

k_{1} x^{2} + k_{2} y^{2 } = 1

can give rise depending on the various possible combinations of signs and values of the constants
k_{1} and k_{2}. The Dupin indicatrix of a surface S at a point P is a device for gaining insight into the
character of the surface near P. The curve of intersection of S and a nearby plane parallel to the
tangent plane at P is approximately similar to the Dupin indicatrix. In the case of a hyperbolic
point the surface S lies on both sides of the tangent plane so we need to intersect the surface with
two planes close to the tangent plane, parallel to it, one on each side. If S lies entirely on one
side of the tangent plane intersection by a single plane is adequate.

Asymptotic directions. A direction at a point P on a surface for which

1) L du^{2} + 2M du dv + N dv^{2} = 0

is called an asymptotic direction. Because the denominator of

is positive definite, the asymptotic directions are also the directions in which k_{n} = 0. At an
elliptic point there are no asymptotic directions, at a hyperbolic point there are two distinct
asymptotic directions, at a parabolic point there is one asymptotic direction and at a planar point
every direction is asymptotic.

Asymptotic lines. A curve on a surface which is tangent to an asymptotic direction at every point is called an asymptotic line. Thus a curve on a surface element is an asymptotic line if and only if at each point of the curve the direction of the tangent to the curve satisfies 1). At a hyperbolic point equation 1) has two real distinct factors of the form Adu + Bdv = 0 which can be regarded as first order differential equations of the asymptotic lines.

Theorem 1. The u and v coordinate curves on a surface element are asymptotic lines if and only if at each point L = N = 0.

References.

1. Graustein. Differential Geometry.

2. Lipschutz. Differential Geometry

3. James/James. Mathematics Dictionary.

4. Struik. Lectures on Classical Differential Geometry.

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