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Three fixed points are given in the plane.

Along with the three given fixed points what is the equation of the parabola with variable fourth point $(x,y)$ and also what geometrical property of the parabola is altered by the variable point not in the red zone defined by Jean Marie?

Let me explain my question:

Three points define a circle of a unique circum-radius. Five points determine a conic of a unique eccentricity.

And by the same token what is the unique property of a parabola through four points?

I remember setting the discriminant to zero in standard form of general conic obtaining two parabolas $y=\pm \sqrt{ax+b}+(cx+d $) with 4 constants but could not readily get to an elegant form in terms of $(x1,y1),(x2,y2),(x3,y3)$, or if possible as a product of two factors.

Thanking you in advance.

Narasimham
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1 Answers1

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This can be recasted into the so-called "Newton's problem" as treated here : http://www2.washjeff.edu/users/mwoltermann/Dorrie/45.pdf where it is shown that a parabola (in fact two of them) can pass through 4 points

Condition ($C_1$): the 4 points form a convex quadrilateral.

As a consequence, let $T$ be the "filled" triangle formed by the three initial points. Then condition ($C_1$) is equivalent to

Condition ($C_2$): [There is a parabola through the 3 first points and the fourth point] if and only if the fourth point is in the complementary set of the forbidden red region represented in the following figure (triangle $T$ + $3$ spiky regions).

enter image description here

Fig. 1: Forbidden regions for the fourth point (in red)._

Remark : I am very indebted to Oscar Lanzi who has pointed an error of mine about condition $(C_2)$ in a previous formulation ; moreover, he has pointed the exceptional case where the points form a parallelogram: in this case, the parabola(s) are degenerated into 2 parallel lines.

$(C_2)$ is equivalent to the following condition:

Condition $(C_3)$: the barycentrical coordinates of the fourth point must have two positive and one negative component.

As a consequence, $(C_3)$ is equivalent to:

Condition $(C_4)$: [There is a parabola through the 3 first points and the fourth point] if and only if

$$\dfrac{\begin{vmatrix}x_4&x_2&x_3\\ y_4&y_2&y_3\\1&1&1\end{vmatrix}\begin{vmatrix}x_1&x_4&x_3\\ y_1&y_4&y_3\\ 1&1&1\end{vmatrix}\begin{vmatrix}x_1&x_2&x_4\\ y_1&y_2&y_4\\ 1&1&1\end{vmatrix}}{\begin{vmatrix}x_1&x_2&x_3\\ y_1&y_2&y_3\\ 1&1&1\end{vmatrix}^3}\color{red}{<0}$$

(Exponent 3 can be suppressed.)

This may be rendered into the form of a symmetric product of determinants:

$$\begin{vmatrix}x_1&x_2&x_3\\ y_1&y_2&y_3\\1&1&1\end{vmatrix}\begin{vmatrix}x_2&x_3&x_4\\ y_2&y_3&y_4\\ 1&1&1\end{vmatrix}\begin{vmatrix}x_3&x_4&x_1\\ y_3&y_4&y_1\\ 1&1&1\end{vmatrix}\begin{vmatrix}x_4&x_1&x_2\\ y_4&y_1&y_2\\ 1&1&1\end{vmatrix}\color{red}{>0}$$

The sign change comes from the fact that some pairs of columns were interchanged to put the determinants into the symmetric form..

Remark about the reference given above: M. Woltermann (Washington and Jefferson College) has undertook a huge task: providing a digital version of the famous book "One Hundred great Problems of Mathematics" originaly published in german in 1932 by Heinrich Dörrie, meanwhile (intelligently) grooming it. This explains in particular the last sentence mentionning an alternative projective method that indeed can be found in the original version of the book p. 208 n°45 here.

A very detailed analysis of the Newton's problem can be found here.

Section under development: I have attempted to develope my own projective proof using the fact that it is always possible to give a parabola the following parametric equations:

$$x=\tfrac{at^2+bt+c}{t-d}, \ \ \ y=\tfrac{et^2+ft+g}{t-d} \tag{1}$$

(a hint for getting intuition about (1): when $t=d$, one has the unique point at infinity of the parabola).

Please note that there are $7$ unknowns, $a,b,c,d,e,f,g$.

Constraining a fourth point to belong to this parabola works on numerical examples but I have some difficulty to find a general justification.

Oscar Lanzi
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Jean Marie
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    Thanks. I shall remain on the question with an edit soon if you please. – Narasimham Aug 25 '20 at 09:23
  • I see you have changed a lot the text of your question. I have to adapt my answer ! – Jean Marie Aug 25 '20 at 09:32
  • So sorry about that ! (Did not know earlier, a similar problem was solved by Newton!) – Narasimham Aug 25 '20 at 09:52
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    You can have the fourth point outside the triangle and still miss having a parabola. Properly, the fourth point should be in one of three region bounded by one side of the triangle and the extensions of the other two sides. If you require at least one parabola to be nondegenerate, you need to exclude a point in each region that would form a parallelogram with the three given points. – Oscar Lanzi Aug 25 '20 at 11:15
  • @Oscar Lanzi : my bad, you are perfectly right. I correct my text. – Jean Marie Aug 25 '20 at 15:15
  • Nice picture! :-) – Oscar Lanzi Aug 25 '20 at 16:14
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    The figure reminds of a warning present in radioactive areas... Thanks again! See my 4th condition using a product of determinants. – Jean Marie Aug 25 '20 at 16:15
  • I had a nicer form for that criterion but my attempt to edit it in epically failed. :-( – Oscar Lanzi Aug 25 '20 at 19:12
  • @Oscar Lanzi It's fine now ! Thank you ! – Jean Marie Aug 25 '20 at 19:32
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    @JeanMarie: Your answer and Oscar lanzi's comments have been illuminating, interesting and stimulating to me. I can accept your answer now to close discussion but like the fruitful discussion to go on. It is not about down/up voting, reputation etc.hope you understand. – Narasimham Aug 25 '20 at 22:02