Inflection Points of Fourth Degree Polynomials
In an article published in the NCTM's online magazine, I came across a curious property of 4^{th} degree polynomials that, although simple, well may be a novel discovery by the article's authors (but see also another article.) Their research began with a suggestion for investigation of the inflection points of 4^{th} degree polynomials from a 2002 issue of Mathematics Teacher, another NCTM publication.
A fourth degree polynomial will be written as
p(x) = m_{4}x^{4} + m_{3}x^{3} + m_{2}x^{2} + m_{1}x + m_{0}.
In the applet and text below, I shall describe the polynomials by their coefficients, with the exponent of x and the index of its coefficient on the left, like this:
4:  m_{4}  
3:  m_{3}  
2:  m_{2}  
1:  m_{1}  
0:  m_{0} 
4^{th} degree polynomials may or may not have inflection points. These are the points where the convex and concave (some say "concave down" and "concave up") parts of a graph abut. The second derivative of a (twice differentiable) function is negative wherever the graph of the function is convex and positive wherever it's concave. The second derivative is 0 at the inflection points, naturally.
If a 4^{th} degree polynomial p does have inflection points a and b, a < b, and a straight line is drawn through
x_{L} < a < b < x_{R}.
Solving equations involving second derivatives of 4^{th} degree polynomials may tax one's desire to see a problem through. Authors McMullin and Weeks used an ingenious device of not solving the equation at all. Instead, knowing that a and b satisfy a quadratic equation, they wrote the equation
p''(x) = 12m_{4}(x  a)(x  b).
The polynomial p(x) can now be expressed from p''(x) by repeated integration:
4:  m_{4}  
3:  2(a + b)m_{4}  
2:  6abm_{4}  
1:  m_{1}  
0:  m_{0} 
Writing an equation of the line L(x) through (a, p(a)) and (b, p(b)) is straightforward. (In the applet, the line is referred to as the "inflection line."). The difference
(*) 

Along with a and b, x_{L} and x_{R}, are the solutions of
p(x)  L(x) = 0.
At this point, McMullin and Weeks proceed with a CAS (Computer Algebra System) that, due to its symbolic manipulation abilities, solves such equations exactly. Among other things, McMullin and Weeks have found that
(1)  
(2)  a + b = x_{L} + x_{R} 
(3)  
(4)  
(5)  The areas of the three regions between the graphs of p(x) and L(x) are in proportion 1:2:1. 
The appearance of the Golden Ratio in (1) is nothing short of surprising. Mathematician are accustomed to finding the famous constant in unexpected circumstances, but every new discovery is a delight not only for the authors but the entire community as well. (1)(4) are of course interrelated. Below, I tackle (1)(5) with simple algebra and polynomial integration. The simplicity of the calculations may, if not shade light on the reasons for the appearance of the Golden Ratio, at least provide a natural formal explanation for its role in the current problem.
The applet helps grasp the situation. It displays a graph of a 4^{th} degree polynomial whose coefficients are controlled by five sliders in the lower left corner of the applet. Each slider sports 3 draggable points: orange and red for 0 and 1, respectively. The locations of these dots define the origin and the unit of measurements for one specific axis. The blue dot then is associated with the value of a coefficient relative to the position of the origin and the unit.
What if applet does not run? 
First let's see how (1) and (2) follow from (*). Since the roots of the 4^{th} degree polynomial
p(x)  L(x) = m_{4}(x  x_{L})(x  a)(x  b)(x  x_{R}).
Thus the coefficients of p(x)  L(x) have the standard representation, viz.,
(**) 

The comparison of coefficients by x^{3} in (*) and (**) shows (2). The comparison of the free coefficients (those by x^{0}) shows that
(6)
x_{L}x_{R} = (a^{2}  3ab + b^{2}).
Together (2) and (6) assert that x_{L} and x_{R} are the roots of the quadratic equation
(7)
x^{2}  (a + b)x  (a^{2}  3ab + b^{2}) = 0.
The quadratic formula applied to (7) gives (1) right away:
(1')
x_{L,R} = ((a + b) ± (a  b)5)/2.
(3) and (4) are easy consequences of (1'), or (1), if you will.
One additional feature that may be observed playing with the applet is that the graph of the difference
a  x_{L} = x_{R}  b,
which is the statement of symmetry. It can also be observed that another interpretation of (2) is the fact that the midpoints of the two intervals
I claim that this is a characteristic property of the direction of the inflection line. More accurately, we have the following
Proposition
Let M(x) be a linear function. Then the graph of
M(x)  L(x) = const.
Proof
The sufficiency (the "if" part) has been shown: (2) is a statement of the claimed symmetry. The necessity (the "only if" part) follows from the observation that all function
(p  M)''(x) = p''(x).
If there are two linear functions M_{1}(x) and M_{2}(x) such that the graphs of both differences
(p(x)  M_{1}(x))  (p(x)  M_{2}(x)) = M_{2}(x)  M_{1}(x)
has the graph that is also symmetric with respect to the same vertical line. But a straight line symmetric in a vertical axis ought to be horizontal, such that necessarily
M_{2}(x)  M_{1}(x) = const.
Integration of the function p(x)  L(x) between x_{L} and a, between a and b, and between b and x_{R} immediately proves (5). In addition, because of the symmetry, the integral from
(x_{R}  x_{L})^{3}(5(b  a)^{2}  (x_{R}  x_{L})^{2}) / 240 = 0,
or
5(b  a)^{2} = (x_{R}  x_{L})^{2},
which is (3) in a different guise.
Remark
As we've seen, the graph of a 4^{th} degree polynomial that has 2 inflection points, can be symmetrized by subtracting the line through the inflection points. Here's a question for further investigation: in the absence of inflection points, can the graph of a 4^{th} degree polynomial be symmetrized by subtracting a linear function?
(The answer is of course, Yes. To see this, note that, whenever the second degree polynomial equation
References
 L. McMullin, A. Weeks, The Golden Ratio and Fourth Degree Polynomials, OnMath Winter 200405, Volume 3, Number 2 (requires subscription)
 L. McMullin, How I Found the Golden Ratio on my CAS, NCAAPMT newsletter, Winter 2005, 67.
Fibonacci Numbers
 Ceva's Theorem: A Matter of Appreciation
 When the Counting Gets Tough, the Tough Count on Mathematics
 I. Sharygin's Problem of Criminal Ministers
 Single Pile Games
 TakeAway Games
 Number 8 Is Interesting
 Curry's Paradox
 A Problem in CheckerJumping
 Fibonacci's Quickies
 Fibonacci Numbers in Equilateral Triangle
 Binet's Formula by Inducion
 Binet's Formula via Generating Functions
 Generating Functions from Recurrences
 Cassini's Identity
 Fibonacci Idendtities with Matrices
 GCD of Fibonacci Numbers
 Binet's Formula with Cosines
 Lame's Theorem  First Application of Fibonacci Numbers
Activities Contact Front page Contents Algebra
Copyright © 19962018 Alexander Bogomolny
70539214