What Is the Inverse of a Circle?

What Is the Inverse of a Circle?

In a discover, this:

On this post we will attempt to originate sense of what we’re seeing, and attempt to know what it blueprint to invert a circle!

  • Inverse of a True Number
  • Inverse of a Advanced Number
  • Conformal Mappings
  • Inverse of a Circle
  • Enriching the Structure of a Circle
  • Rotations on the Riemann Sphere
  • Digging Deeper
  • Contact me

For any number a we mediate the inverse a^{-1} as whichever number yields:

a a^{-1} = 1

For proper numbers that is inconspicuous:

a^{-1} = frac{1}{a}

Qualitatively, the inverse of a huge number is a minute number and vice versa. There are two special numbers, a = 1 and a = -1, that are their maintain inverses. We call these fastened aspects below inversion.

There are two extraordinary aspects at a = 0 and a = infin, such that:

frac{1}{0} = infin


frac{1}{infin} = 0

Now, infin is rarely any longer truly a proper number and you might perchance well well additionally in most cases get into peril treating it admire one, but for our purposes at the present time we’re going to true faux that it is.

A flowery number is in overall written as text{p} = a + bi. It has a dimension given by:

|text{p}| = sqrt{a^2 + b^2}

And an attitude theta_{text{p}} given by:

theta_{text{p}} = tan^{-1}left(frac{b}{a}true)

Its inverse is defined true admire ahead of:

text{p}^{-1} = frac{1}{a+bi}

Nonetheless having a advanced number in the denominator of a bit is exhausting to reason about. How operate we pull that i into the numerator so we’re going to originate sense of it?

A conventional desire is to multiply both the numerator and denominator by the advanced conjugate, a-bi:

text{p}^{-1} &= frac{1}{a + bi} \
&= frac{1}{a + bi} cdot frac{a-bi}{a-bi} \
&= frac{a-bi}{a^2 -abi + abi – b^2 i^2} \
&= frac{a-bi}{a^2 + b^2}

text{p}^{-1} = textcolor{orange}{frac{1}{a^2 + b^2}} textcolor{ff00ff}{(a – bi)}

These ingredients one at a time management the dimensions and attitude of text{p}^{-1}:

The textcolor{orange}{frac{1}{a^2 + b^2}} component blueprint the novel dimension is the inverse of the extinct dimension, true as with proper numbers.

The textcolor{ff00ff}{a – bi} component blueprint the direction is the identical, but mirrored over the actual axis.

Now that we now hang some concept of what to request, we’re going to play around with a advanced number inverter to are attempting to produce some instinct:

Can you visually verify that huge numbers change into minute and that each number is mirrored over the actual axis?

One frosty property of advanced inversion is that every level inner the unit circle gets inverted outdoor of the circle and vice versa, but every level on the unit circle stays on the unit circle.

Glance the 2 fastened aspects at 1 + 0i and -1 + 0i. It is some distance true that fastened aspects can most efficient happen on the proper axis, since inversion mirrors over the actual axis.

As a designate of things to attain: What happens whereas you development your cursor alongside the vertical line with proper cost 0.25? What path does the inverse designate out?

To admire inversion of a circle we now need to understand a bit bit about conformal mappings.

For our purposes, a mapping Mu is a feature that turns one advanced number into yet some other:

As an illustration:

Mu_{double}(a+bi) = 2a + 2bi


Mu_{replicate}(a+bi) = a – bi

There will doubtless be a convenient shorthand for mappings, mapsto, which is pronounced “maps to” and is frail admire this:

a + bi mapsto a^2 + 3bi

Which is ready to also be frail to recount a mapping or illustrate order properties of a order mapping.

Conformal mappings hang a special simplifying property: they in the neighborhood retain angles.

In the neighborhood preserving angles blueprint that whereas the dapper-scale structure might perchance well additionally honest trade, the minute-scale structure stays exactly the identical but for scaling and rotation. Which blueprint any two traces which intersect at theta ahead of the mapping will mute intersect at theta after the mapping.

Example of a Conformal Mapping

This also implies that any two traces that are parallel ahead of the mapping are mute parallel after the mapping, no much less than in the neighborhood. Despite the indisputable truth that the mapping might perchance well additionally honest warp straight traces into curves, whereas you zoom in cease ample to any minute neighboring line segments they are mute in the neighborhood parallel.

Acknowledged yet some other contrivance: Little squares ahead of the mapping will mute be cramped squares after the mapping, though they’ll be shifted, scaled, and turned around. Squares is rarely any longer going to warp into rectangles or diamonds.

From this falls an an identical definition of conformal: Circles ahead of the mapping dwell circles after the mapping, but for scale and translation. The sizes and locations of the circles might perchance well additionally honest trade, but circles by no blueprint get squished into ovals. These definitions will all be crucial later.

To test our working out let’s scrutinize a authorized mapping that transforms aspects on the Earth onto aspects in the advanced plane, the Mercator Projection.

We call it a projection because we’re particularly mapping from a sphere to a plane, but it no doubt’s true a order form of mapping.

If the longitude is represented as theta and latitude is represented as phi, then the Mercator Projection is:

M(theta, phi) = Rtheta + R lnleft[tan{left(frac{pi}{4} + frac{phi}{2} right)} right]i

Example of the Mercator Projection

Is this mapping conformal? We are able to crawl a series of tests:

  1. Lines of constant Latitude (admire the Equator, Tropic of Capricorn, and loads of others) are all mutually parallel on the ground of the Earth. So too, on this projection. checkmark
  2. Lines of constant Longitude (admire the Top Meridian) are all mutually parallel on the ground of the Earth. All once more, this will doubtless be true on this projection. checkmark
  3. The put Latitude and Longitude traces intersect, they operate so at 90stage angles. Here will doubtless be true after projection. checkmark

From these tests we’re going to infer that the projection is conformal!

There is yet some other projection known as the Stereographic Projection which is extra functional for inverting circles:

Example of the Stereographic Projection

The Stereographic Projection unwraps the Earth onto the 2D plane by placing the South Pole in the initiating put and stretching out the North Pole to r = infin in step with:

S(theta, phi) = 2Rtan{left(frac{phi}{2}true)} (cos(theta) + sin(theta) i)

Is the Stereographic projection conformal?

  1. Lines of constant Latitude are mutually parallel: checkmark
  2. Lines of constant Longitude are all mutually parallel: checkmark
  3. Lines of Latitude and Longitude are perpendicular: checkmark

So sure, this projection will doubtless be conformal.

A of conformality is that any circle on the sphere of the Earth maps to a circle on the 2D plane. Here is trivially true of the Equator, but how about the Top Meridian, which has change into an infinite line?

Effectively, the Top Meridian passes through the North Pole which maps to the level at infin. Which blueprint the Top Meridian, when projected onto the advanced plane, wishes to be a circle which contains the level infin, which blueprint it wishes to be a circle of radius infin, which appears admire a line!

We might perchance well additionally also see at the Stereographic projection in the reverse direction: Let’s wrap the entire advanced plane onto a sphere such that the South Pole of the sphere touches the initiating put and the North Pole represents the entire aspects that are infinitely some distance-off from the initiating put.

If we’re careful, we’re going to operate this in honest such a vogue that the unit circle in the advanced plane maps to the sphere’s equator. This object is is concept as the Riemann Sphere:

Image of the Riemann Sphere

When you flip backward and forward between the Riemann Sphere and the Stereographic projection report you might perchance well well additionally presumably convince your self that they are truly two descriptions of the identical mapping.

Qualitatively: All the minute advanced numbers (magnitude <1) are in the Southern hemisphere and all the big complex numbers (magnitude >1) are in the Northern hemisphere.

The ground of the Riemann Sphere contains the entire advanced plane so it is cheap for us to query: What does advanced inversion see admire on this ground?

The answer is inconspicuous to visualize:

Complex Inversion on the Riemann Sphere

A degree text{p} on the advanced plane is first mirrored across the equator to text{p}’. This transforms the magnitude from one thing minute to one thing dapper.

Then text{p}’ is mirrored across the actual meridian, finally arriving at text{p}^{-1}.

These two mirroring operations correspond exactly with the 2 ingredients of advanced inversion, derived above:

text{p}^{-1} = textcolor{orange}{frac{1}{a^2 + b^2}} textcolor{ff00ff}{(a – bi)}

With this novel report in ideas, attempt to visually streak through the identified special cases from earlier:

  • Fastened aspects at 1 and -1
  • 0 mapsto infin
  • i mapsto -i

Now that we now hang received some 3D instinct for advanced inversion on the Riemann Sphere, we’re ready to tackle the bulky self-discipline.

Given that the Stereographic Projection is conformal, we all know that a circle on the advanced plane maps to a circle on the Riemann Sphere and vice versa.

Reflection across the unit circle preserves the form of the circle, it true strikes it from one hemisphere to the diversified.

By that identical common sense, mirroring across the proper meridian also preserves the form of the circle.

Lastly, mapping that circle attend to the advanced plane is performed utilizing the inverse of the Stereographic Projection, which need to also be conformal.

Circle Inversion on the Riemann Sphere

In transient:

The inverse of a circle on the advanced plane wishes to be yet some other circle on the advanced plane.

Qualitatively: Minute circles cease to the initiating put invert to huge circles some distance from the initiating put and vice versa. Circles above the actual line invert to circles below the actual line and vice versa.

The symmetry of the Riemann Sphere now lets us identify some special cases:

  1. A circle centered on 1 or -1 is its maintain inverse
  2. A circle centered on i inverts to a circle with the identical radius, centered on -i

Factors 1 and a pair of are without say confirmed by simulation, but now we now hang a rich visible working out of why they happen:

Nonetheless wait, placing the cursor at +1 does no longer create a circle which is its maintain inverse! Now we need to sprint out to one thing admire +1.1 for the inverse to compare. What offers?

The gap here is that a circle centered on +1 on the Riemann Sphere does no longer translate to a circle centered on +1 in the advanced plane:

Circle Inversion on the Riemann Sphere

When projected onto the advanced plane, the circle pictured need to hang +1 and its left side need to tumble between 0 and 1, but its true side might perchance well additionally extend very some distance out into extremely dapper numbers. The increased the radius of the circle, the increased the offset between the facilities.

Here is a subtle level that’s price rehashing. The exercise we did above the put we reflected a circle across the equator (unit circle) and nonetheless across the actual meridian (proper axis), all of that’s true both for the aspects on the circumference of the circle and for the midpoint of the circle once it lives on the Riemann Sphere. We true need to do not omit that whereas the Stereographic Projection preserves the shapes of circles, it does warp the gap inner them such that their midpoints are no longer in overall preserved!

Now select into consideration a circle that touches the initiating put:

Circle Inversion on the Riemann Sphere

A circle that touches the initiating put inverts to a circle that passes through infin!

In expose for a circle to head through infin, it need to stretch to the fringe of the advanced plane. It wishes to be a circle of countless radius, which appears admire a straight line!

We saw this phenomenon as a special case earlier with the Top Meridian and the Stereographic Projection, but here it appears as a authorized rule: any circle that touches the initiating put will invert to a line, no longer true meridians.

On this case we all know that line passes through +1 and that it approaches infin in a contrivance parallel to the imaginary axis, so this straight line wishes to be a vertical line that passes through +1. Scroll attend as a lot as the simulation to verify this to your self!

Let’s set up a bow on this topic by figuring out a number of extra fastened aspects, or might perchance well additionally honest mute we call them fastened circles?

  • The unit circle inverts to itself
  • The proper axis inverts to itself
  • Any circle centered on 1 or -1 (on the Riemann Sphere, no longer the advanced plane) inverts to itself. The imaginary axis is as a result of this truth its maintain inverse

So inversion of a circle has one countless household of fastened circles: those parallel to the imaginary axis, after which two special outliers which happen to also be fastened.

To illustrate that our enter circle is spinning clockwise.

We are able to then expose that every person aspects in the neighborhood on the apt hand side of the circumference are inner the circle, so we will paint them orange.

On the advanced plane now we hang filled-in circles with arrows to show direction:

On the Riemann Sphere this might perchance well additionally honest see admire a dome:

Filled-in Circles on the Riemann Sphere

Fooling around, we’re going to uncover about that circles centered on 0 are flipped inner out! One more contrivance to claim that is that their plug direction is reversed.

The filled-in unit circle is rarely any longer the equator, it is the entire Southern Hemisphere on the Riemann Sphere. Below inversion it turns into the entire Northern Hemisphere! So this circle is rarely any longer its maintain inverse.

The proper axis is rarely any longer the actual meridian, it is the entire hemisphere closer to us in the contrivance, which inverts to the entire hemisphere some distance from us. This circle too, is rarely any longer a special “fastened circle”.

Nonetheless the imaginary axis is mute its maintain inverse! The half-plane to the apt of the imaginary axis styles the Jap Hemisphere on the Riemann Sphere, which inverts to itself.

By alongside side structure to the circle, the inversion feature has misplaced two of its fastened circles!

We’ve been visualizing advanced inversion as two reflections but we might perchance well additionally also stare it as a single 180stage rotation about the [-1, 1] axis:

Riemann Sphere Rotation

Hurry ahead and verify a number of identities to convince your self that all of them figure out identically:

  • Fastened aspects at 1 and -1
  • 0 mapsto infin
  • i mapsto -i
  • imaginary axisimaginary axis
  • unit circleunit circle but no longer if it is filled in
  • proper axisproper axis but no longer if it is filled in

Having a stare upon it this contrivance provides novel insight. Rotation about the [-1, 1] axis might perchance well additionally be known as rotation in a plane perpendicular to the [-1, 1] axis. One such plane passes through the entire imaginary axis.

So in a colloquial sense, advanced inversion is a 180 stage rotation in the imaginary plane. Any circle parallel to that plane (true see on the Riemann Sphere) need to invert to itself because circles are invariant to rotation. Circles perpendicular to that plane will most efficient invert to themselves in the occasion that they lack handedness.

We’ve also learned a tantalizing novel request! What happens if we had been to rotate about the [-i, i] axis, aka in the actual plane?

Riemann Sphere Rotation 2

Effectively, dapper numbers mute change into minute numbers and vice versa, but it no doubt’s admire we’re reflecting over the imaginary axis in desire to the proper axis.

This implies some diversified definition of advanced inversion! Sooner than, we had been utilizing:

text{p}^{-1}_{textcolor{green}{imaginary}} = textcolor{orange}{frac{1}{a^2 + b^2}} textcolor{ff00ff}{(a – bi)}

Nonetheless this novel rotation corresponds to:

text{p}^{-1}_{textcolor{blue}{proper}} = textcolor{orange}{frac{1}{a^2 + b^2}} textcolor{ff00ff}{(-a + bi)}

Which is rarely any longer an operation that I’ve considered ahead of! It fashions a Bizarro-World advanced plane the put the special properties of the imaginary axis were taken away and as a replace granted to the actual axis.

From this we’re going to simulate a special but mute perfectly cheap definition of the inverse of a circle!

We are able to search out out about in the simulation and on the Riemann Sphere that for this operation:

  • The fastened aspects are pm i
  • The fastened circle is the proper axis and any diversified circle centered on pm i

That brings us to our final contender, a rotation about the [0, infin] axis, or a rotation in the plane defined by the unit circle:

Riemann Sphere Rotation 3

Can you uncover about what this operation does to advanced numbers? Broad numbers maintain huge and minute numbers maintain minute. This form of advanced inversion is most efficient altering the direction of advanced numbers.

On the advanced plane, all we’re doing is rotating 180 stage around the initiating put!

  • Our fastened aspects are 0 and infin
  • Our fastened circle is the unit circle and any diversified circle centered on the initiating put

By inspection we’re going to write out this inversion system as:

text{p}^{-1}_{textcolor{red}{unit}} = textcolor{ff00ff}{-a – bi}

The three definitions that we now hang attain to all see outlandish but now we all know that they are actually three contributors of the identical household: They are all 180 stage rotations of the Riemann Sphere!

I hope this post conveys a bit little bit of what math can in actuality feel admire in the occasion you true proceed exploring. It be no longer so great about equations and manipulation of symbols as it is about asking “what if” after which following through.

Talking of, here’s many strategies you might perchance well well additionally dive deeper:

What does it point out to rotate by 90 stage on the Riemann Sphere? Is this a neatly-recognized operation viewed in a novel light, or one thing novel? Can you rep an order system for it in all three planes?

What does it point out to rotate the Riemann Sphere by any arbitrary phi? Can you rep an order system, maybe making use of the advanced exponential re^{itheta}?

Can you generate one of the most inversion formula above utilizing most efficient the diversified two? What implications does this hang?

On the Riemann Sphere, what happens whereas you replicate an outlandish selection of times in desire to a honest number? What are the fastened aspects and circles of such an operation? What about with filled circles?

Are there any squares which invert to themselves? Does it topic whereas you true use the four corners of the square vs every level alongside its edges?

The self-discipline of math we now were taking part in in is is concept as Advanced Evaluation. You will doubtless be ready to also learn extra by deciding on up a e-book, taking part in around online, or true see some movies on Youtube!

I’m on Twitter at @mferraro89 and you might perchance well well additionally shoot

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Ava Chan

Ava Chan

I'm a researcher at Utokyo :) and a big fan of Ava Max