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What is the inverse of a circle?

mattferraro.dev

31–40 of 51 posts

Re: What is the inverse of a circle?

#31
post #14

The answer is “it depends on how you defined the operation that acts on the circle”

As long as, C*Cinv = I, where C is the circle, Cinv is the inverse of circle, and I the identity. You're right. C, I, and * are entirely up in the air.

An inverse is pretty easy to generalize, and it need not rely on multiplication or the identity.

Basically: Given a function `f`, then the function `g` is the inverse of `f` if `g(f(x)) = x`.

Re: What is the inverse of a circle?

#32

This article is interesting but not rigorous I think. * The inverse of a geometric shape makes no sense. We only inverse operations. * aa^-1 = 1 only if you consider the multiplication over reals. * 1/0 is not equal to infinity. Because the article is interesting but some people might be put off by the first few sentences, I suggest to had a disclaimer that this article lean on edutainment to the detriment of rigorou…

It depends on your definitions and which mathematical objects you are working with. The notions in the blog post are not something the author invented themselves.

For example you can find Riemann Sphere in wikipedia - https://en.wikipedia.org/wiki/Riemann_sphere

Re: What is the inverse of a circle?

#34

This article is interesting but not rigorous I think. * The inverse of a geometric shape makes no sense. We only inverse operations. * aa^-1 = 1 only if you consider the multiplication over reals. * 1/0 is not equal to infinity. Because the article is interesting but some people might be put off by the first few sentences, I suggest to had a disclaimer that this article lean on edutainment to the detriment of rigorou…

Your assertions are simply not true in the context of complex analysis. It is common to use "inverse" to refer to the multiplicative inverse as shorthand (though potentially confusing). a a^-1 = 1 is absolutely and uncontroversially applicable to any complex number. It is common and natural to extend to complex plane to include a single point at infinity (known as the extended complex plane, see e.g. https://mathworld.wolfram.com/ExtendedComplexPlane.html ). When you are working in the extended complex plane, 1/0 does equal infinity.

Re: What is the inverse of a circle?

#36
post #14

Earlier quoted context omitted.

As long as, C*Cinv = I, where C is the circle, Cinv is the inverse of circle, and I the identity. You're right. C, I, and * are entirely up in the air.

An inverse is pretty easy to generalize, and it need not rely on multiplication or the identity. Basically: Given a function `f`, then the function `g` is the inverse of `f` if `g(f(x)) = x`.

It is easy to generalize, but it is not being generalized here.

Re: What is the inverse of a circle?

#37

I did some wacky inside-out pics of planets years ago, inspired by the inside-out Mandelbrot set looking like a leaf/teardrop. https://www.adamponting.com/inside-out/ Related: Not Knot , 1991 Thurston-ish short film about knots and knot complements - "the space where the knot isn't". https://www.youtube.com/watch?v=zd_HGjH7QZo https://en.wikipedia.org/wiki/Not_Knot https://en.wikipedia.org/wiki/Knot_complement

Is it just me, or does it look like black holes hovering over the planets? I guess that is to be expected, if a planet with black background is used. Pseudo-scientific question: Could black holes have anything to do with something being inverted?

Re: What is the inverse of a circle?

#38

In polar coordinates a circle can be defined by all points where r = a. The inverse would be all points where r doesn't equal a.

I interpreted the question myself from another angle: a circle is a function where every f(x) is an equal linear distance to an arbitrary fixed point z. So, the "inverse" to this function could be a function where every f(x) must have a different linear distance to z.

Yep, this would have the same effect. Both of these define all of the points that do not describe the circle.

However, as someone said above, f() is the inverse of g() if g(f(x)) = x. When put into practice, this means that the inverse is the reflection of the original function over y = x.

However, there's one problem with looking at the problem this way: A circle is NOT a function. Therefore, it does not have an inverse as we are thinking of it. A circle can be described by two functions, and both of these inverses combine to form the same circle. So, the inverse of a circle is (sort of) itself.

Re: What is the inverse of a circle?

#39

I did some wacky inside-out pics of planets years ago, inspired by the inside-out Mandelbrot set looking like a leaf/teardrop. https://www.adamponting.com/inside-out/ Related: Not Knot , 1991 Thurston-ish short film about knots and knot complements - "the space where the knot isn't". https://www.youtube.com/watch?v=zd_HGjH7QZo https://en.wikipedia.org/wiki/Not_Knot https://en.wikipedia.org/wiki/Knot_complement

Is it just me, or does it look like black holes hovering over the planets? I guess that is to be expected, if a planet with black background is used. Pseudo-scientific question: Could black holes have anything to do with something being inverted?

A black hole is just an object with so much dense mass that its Schwarzschild radius[1^] extends outside of its body. Everything has a Schwarzschild radius.

[1^]: https://en.wikipedia.org/wiki/Schwarzschild_radius

Re: What is the inverse of a circle?

#40
post #23
post #14

Earlier quoted context omitted.

As long as, C*Cinv = I, where C is the circle, Cinv is the inverse of circle, and I the identity. You're right. C, I, and * are entirely up in the air.

In general for inversion, we have object A (argument) and object I (identity) and a function F of two arguments, so we have equations: `F(A, X) == I, F(A, I) == I, F(X, I) == I, A != I, A != X`, where A, I, and X are objects in the same category, i.e. they must be circles `(x² + y² == r²)`. If F is defined as `ra•rx`, then `ri == 1`, and inverse will be `rx = 1/ra`. If F is defined as `ra + rx`, then `ri == 0`, and i…

Correction: `F(A, I) == A, F(X, I) == X`.
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