A Canonical Conical Function
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Transcript of A Canonical Conical Function
D. N. Seppala-HoltzmanSt. Joseph’s College
A Canonical Conical FunctionTo appear in The College Mathematics
JournalIntended for a general audienceThis presentation can be downloaded from
the “downloads” page of: faculty.sjcny.edu/~holtzman
EccentricityAs the angle of tilt increases, the ellipses
become flatter and more elongatedMathematicians say the that the eccentricity
is increasingThis will be defined later
Eccentricity IIAs the angle of tilt increases, the hyperbolas
will open up more and moreAgain, the eccentricity is increasingI still owe you a definition
The Conic SectionsThe circle, ellipse, parabola and hyperbola
make up the family of conic sectionsThese were studied by the ancient Greeks
ApolloniusApollonius (262 – 190 B.C.) wrote a treatise
on them
Euclid Apollonius √ Pythagoras
Foci and VerticesConics have important points called foci and
verticesWe will need these to define eccentricity
Let us start with the ellipseHammer two nails into a boardTake a piece of string whose length is greater
than the distance between the stringsTie each end to one of the nailsPull the string taut with a pencil and draw a
curve that keeps the string taut at all timesThis will produce an ellipse
The foci of an ellipseThe two nails represent the foci of the ellipseAn ellipse is defined to be the set of points in
the plane the sum of whose distances to two fixed points (the foci) is a constant
Note that the length of the string is this constant distance
Foci and vertices of an ellipseThe foci of an ellipse are equidistant from the
center, lying on its central axisThe vertices of an ellipse are those two points
where the ellipse intersects its central axisTraditionally, we call the distance from the
center to either focus “c” and the distance from the center to either vertex “a”
Eccentricity of an ellipseThe eccentricity, e, of an ellipse is defined to be
e = c/aAs c < a, we have 0 < e < 1 The closer e gets to 1, the more elongated the ellipse
becomesThe closer e gets to 0, the more circular it becomesThe limiting case occurs when the foci coincide with
the center and the result is an actual circle. Circles have eccentricity, e = 0
The focus and vertex of a parabolaA parabola has a single focusThis is the unique point on the central axis
with the property that, if the parabola were a mirror, every light ray emitted from the point would reflect off the curve and travel parallel to the axis
Conversely, all in-coming rays parallel the axis would pass through the focus
The vertex is the point where the parabola crosses its axis
The focus of a parabola IIThis is why car headlights have parabolic
reflectors around the light source which lies at the focus
This is also why radio telescopes and dish antennae are parabolic bowls with the receiver at the focus
Eccentricity of the parabolaThe eccentricity of any parabola is equal to 1
The foci and vertices of a hyperbolaA hyperbola is defined to be the set of points
in the plane the difference of the distances to two fixed points is a constant
Recall that in the elliptical case, the sum of the distances was held constant
The two fixed points are the foci of the hyperbola
The points where the hyperbola intersect its central axis are the vertices
Foci and vertices of a hyperbolaThe foci and vertices of a hyperbola are
equidistant from the center, lying on its central axis
Traditionally, we call the distance from the center to either focus “c” and the distance from the center to either vertex “a” just as in the elliptical case
The eccentricity of a hyperbolaThe eccentricity, e, of a hyperbola is defined
to the quotient e = c/a just as it is in the elliptical case
As c > a, we have e > 1 for all hyperbolas
Conic eccentricities summarizedCircle: e = 0Ellipse: 0 < e < 1Parabola: e = 1 Hyperbola: e > 1
Why all the fuss about eccentricity?Any two conics with the same
eccentricity are similarThus, any two circles are similar as they all
have e = 0Likewise, any two parabolas are similar since
they all have e = 1For ellipses and hyperbolas, similarity
classes vary with e
What is similarity, anyway?Two shapes are similar if one can be scaled
up or shrunk down so that it can be placed over the other, matching it identically
Similarity of circlesClearly, given two circles, one could increase
or decrease the radius of one of them, making the two identical
Here, the radius is the scaling factor
Similarity of parabolasLikewise, one could increase or decrease the
distance from the vertex to the focus of one parabola to make it identical to any other parabola
Here, the distance from focus to vertex is the scaling factor
Similarity leads to constantsAny geometric construct on a similarity class
that is independent of the scaling factor, leads to a constant for that class
For example, consider the circleTake any circleCompute the ratio of the circumference divided
by the diameter. Note that the scaling factor, R, cancels. The result is a very famous constant:
2
2
C R
D R
Two ConstantsPursuing this pattern Sylvester Reese and
Jonathan Sondow made a pair of geometric constructs, one for all parabolas and one for a special hyperbola
These gave rise to two constants:The Universal Parabolic ConstantThe Equilateral Hyperbolic Constant
Two Constants IITheir respective values
were:
2 ln(1 2)
2 ln(1 2)
Holy Cow!The similarity of these two constants was
either an indicator of a profound mysterious truth or a mere coincidence
No one knows which
The ProblemTrying to get to the bottom of this question
one faces a big problem:The two constructions yielding the two
constants are incompatibleThe one carried out on the parabola could not
be done on the hyperbola and vice versa
A Unifying Construction is NeededA unifying construction that can be carried
out on all conics yielding a value that depends only upon the eccentricity is called for
One would want this construction to yield a smooth, continuous function of e
A Canonical Conical FunctionMotivated by this need, I created what I call
(with a nod to Dr. Seuss) a Canonical Conical Function
This function has the desired properties just discussed
Latus RectumTo define the function, I must first define a
line segment that all conics have: the latus rectum
Latin for “straight side,” the latus rectum is chord passing through a focus and orthogonal to the axis
Canonical Conical Function IIFor any conic, let A denote the area of the
region bounded by the curve and its latus rectum
Let L denote the length of its latus rectum We define our Canonical Conical Function by:
2
A
L
Canonical Conical Function IIIThis construction generates a smooth,
continuous function depending only on eIt is calculated for each class of conic and is
defined piecewiseWe denote this function by C(e)
The Circle (e = 0)We compute the value of A/L2 for the circle
and we get the value:
As the eccentricity of the circle is 0, we assign:
8
(0)8
C
The Ellipse (0 < e < 1)Doing the same construction for an ellipse,
we get the value of our function solely in terms of e:
2 3 2
2 2
1 2 2 2arcsin( ) 1( )
8(1 )
e e e e eC e
e
The Parabola (e=1) We compute the value of A/L2 for the
parabola and we get the value: 1/6As the eccentricity of a parabola is 1, we
assign:
1(1)
6C
The Hyperbola (e > 1)We compute the value of A/L2 for each
hyperbola and we get the value:
3 2 2
2 2
[ln( 1) 1]( )
4( 1)
e e e e eC e
e
Canonical Conical Function IVThus, C(e) is defined by the exact same
construction for each conic but has a different algebraic expression in each of the four categories
Amazingly, this function is smooth and continuous
Behold its graph on the next slide
ConclusionAnd so we have achieved what we set out to
doWe have found a single construction that can
be carried out on every conic which yields a smooth, continuous function, uniting the entire family of conics
Thank you for listening