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ORTHOGONAL PURSUIT CURVE, DUPORCQ CURVE
Curve studied by Duporcq and Mannheim in 1902, Balitran in 1914, Egan in 1919, Masurel in 2014.
Ernest Duporcq (18731903): French mathematician. Other name: crab curve. See also: Walter Wunderlich, Über die Hundekurven mit konstantem Schielwinkel, Monatshefte für Mathematik, 1957, Volume 61, Issue 4, pp 299303. 
Differential system: , yielding, when , , i.e. . 
The orthogonal pursuit curves are the trajectories of a point M the motion of which is always in a direction perpendicular to that of the other moving point M_{0}
(the pursuee), the two moving points having proportional speeds .
Given the trajectory of M_{0}, the trajectory of M is therefore defined in this case by the fact that (MM_{0} ) is perpendicular at M to its trajectory, and the curvilinear abscissa of M is proportional to that of M_{0}: . One can imagine a crab at M always walking face to face with the pursuee M_{0} and these curves could be called "(pursuit) curves of the crab". 
I) In the case where M_{0} has a linear movement, the trajectories of the "pursuer" are called "Duporcq curves".
Egan proved that the vector then has the same movement as a body in Newtonian attraction, and therefore describes a conic with eccentricity e (hence the choice of the letter e for the ratio between the speeds, instead of the more classic k).
First case: , elliptic case.
Cartesian parametrization: , , .
Curvilinear abscissa: . 
Opposite, in blue, the elliptic movement of the vector is indicated.
The Duporcq curve is, in this case, the image by a scaling in one direction with ratio of an elongated cycloid with ratio . NOTA 1: it is the only scaling that allows to integrate the curvilinear abscissa thanks to elementary functions. NOTA 2: in the formulas above, and as it can be noticed in the animation opposite, the speeds of M and M_{0} are proportional, but not constant. 
Second case: , parabolic case.
a) the pursuer and the pursuee have, at a given time, their speeds in the same direction: the pursuer evidently follows a line parallel to that followed by the pursuee.
b) the pursuer and the pursuee have, at a given time, their speeds in opposite directions.
Cartesian parametrization: ,,
Curvilinear abscissa: . 
Opposite, in blue, the parabolic movement of the vector is indicated.
The Duporcq curve is none other, in this case, than the Tschirnhausen cubic. NOTA: the movement of M is the composition of a parabolic motion and a translation.

Third case: , hyperbolic case.
a) the pursuer and the pursuee are at the same point at a given time: they follow to secant lines.
This case is excluded in the following.
b) The pursuer and the pursuee have, at a given time, their speed in the same direction:
Cartesian parametrization:, ,
Curvilinear abscissa: . Opposite, in blue, the hyperbolic movement of the vector is indicated. 
c) the pursuer and the pursuee have, at a given time, their speeds in opposite directions:
Cartesian parametrization: , , .
Curvilinear abscissa: . Opposite, in blue, the hyperbolic movement of the vector is indicated. 
REMARK: Mannheim proved that the Duporcq curve with parameter e is the roulette with linear base of the pole of the Sturm spiral satisfying .
II) Case where the pursuee describes any curve:
Vectorial differential equation: , that translates into the differential system:
( = pursuee, = pursuer, ). 
For a pursuee on the circle with centre O and radius R, we get the differential system: which enables to draw the curves thanks to a software.
An example with k = 1. 
An example with k = 1/3. 
The shadowing curve associated to the point describing a circle (in blue below) and to a point on the circle (the tree) provides a circular special case of the curve of the pursuer associated to a circle.
If is the radius of the pursuee's circle and R that of the pursuer's circle, we have . Below, an example with k = 1/2, . 

VARIATION (based on an idea of Alain Esculier): the speed of the pursuer is no longer proportional to that of the pursuee, but to its distance from it.
Vectorial differential equation: , which translates into the linear differential system: .
Equations of the movement of the pursuer: . 
I) Linear pursuee.
For a pursuee (vt, 0), equations of the movement of the pursuer passing by (0, b):
(a = v/k). Therefore, it is a trochoid with ratio , which is a cycloid if . 

II) Circular pursuee.
For a pursuee , equations of the movement of the pursuer passing by (0, a) when :
. It is an epitrochoid when and a hypotrochoid when , epicycloid and hypocycloid when a = R. The parameter q is equal to . 

When , we get: , which is none other than an involute of a circle (more precisely, involute of the circle described by the pursuee). 
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© Robert FERRÉOL
2017