Is it possible to define this explicitly?

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Discussion Overview

The discussion revolves around the possibility of defining the equation of a circle explicitly in terms of the variable y. Participants explore whether it is feasible to express y as a function of x, given the inherent multi-valued nature of the relation defined by the equation.

Discussion Character

  • Exploratory
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant presents the equation of a circle and questions the feasibility of defining y explicitly, suggesting it may be a mathematical impossibility.
  • Another participant argues that the original equation explicitly defines a circle and questions the meaning of "define explicitly," implying that a function y(x) cannot be formed since multiple y values correspond to a single x value.
  • Several participants express skepticism about the notion that being multi-valued precludes an explicit definition, proposing that it is still possible to manipulate the equation to express y in terms of x, albeit with two solutions for y.
  • One participant suggests that a clever theory might eventually provide a solution to the problem, indicating an openness to future developments in understanding.
  • Another participant mentions the possibility of completing the square as a method to approach the problem, although this suggestion does not resolve the core question of explicit definition.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the definition of "explicitly" or the implications of multi-valued functions. Multiple competing views remain regarding the possibility of defining y explicitly in the context of the given equation.

Contextual Notes

Participants express uncertainty about the definitions and implications of explicit functions versus relations, and there are unresolved mathematical steps regarding the manipulation of the equation.

Nano-Passion
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(x-3)^2 + (y-2)^2 = 1

With some rearranging I get

y(y-4) = -x^2+6x-13

Is it possible to define this explicitly in terms of y? It looks like a mathematical impossibility to define it explicitly in terms of y but I am hoping there might be some analytical technique for this.
 
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What does "define explicitly" even mean? The formula at the start explicitly defines a circle.
Do you want a function ##y(x)##? Then it should be easy to see the answer is no: for every x there are two values of y.
 
pwsnafu said:
What does "define explicitly" even mean? The formula at the start explicitly defines a circle.
Do you want a function ##y(x)##? Then it should be easy to see the answer is no: for every x there are two values of y.

Define the function explicitly in terms of y.

And okay.
 
I don't see why not being single-valued means it's impossible to define something explicitly.

##(y-2)^2 = \text{something}##
##(y-2) = \pm \sqrt{\text{something}}##
...
 
AlephZero said:
I don't see why not being single-valued means it's impossible to define something explicitly.

##(y-2)^2 = \text{something}##
##(y-2) = \pm \sqrt{\text{something}}##
...

Well in this case it is more like (y^2-4y) = something. so it is bit more complicated. Who knows, maybe one day some genius might develop a clever theory to solve it.
 
AlephZero said:
I don't see why not being single-valued means it's impossible to define something explicitly.

##(y-2)^2 = \text{something}##
##(y-2) = \pm \sqrt{\text{something}}##
...

This is why I asked what "define explicitly" means.

Nano-Passion said:
Well in this case it is more like (y^2-4y) = something. so it is bit more complicated. Who knows, maybe one day some genius might develop a clever theory to solve it.

:confused: How did you get that?
Edit: And why not just complete the square?
 
Nano-Passion said:
(x-3)^2 + (y-2)^2 = 1

With some rearranging I get

y(y-4) = -x^2+6x-13

Is it possible to define this explicitly in terms of y?
You can solve the first equation for y in terms of x. However, the equation defines a relation between x and y, but doesn't represent a function. Some values of x map to two different y values.

Having said all that, it's pretty simple to solve for y. Leave the terms in y on one side, and move the terms in x to the other side. Do not expand the (y - 2)2 term. Then take the square root of both sides, remembering to use ± for the two solutions.
Nano-Passion said:
It looks like a mathematical impossibility to define it explicitly in terms of y but I am hoping there might be some analytical technique for this.
 
Mark44 said:
You can solve the first equation for y in terms of x. However, the equation defines a relation between x and y, but doesn't represent a function. Some values of x map to two different y values.

Having said all that, it's pretty simple to solve for y. Leave the terms in y on one side, and move the terms in x to the other side. Do not expand the (y - 2)2 term. Then take the square root of both sides, remembering to use ± for the two solutions.

Oh, now I see what Alephzero was saying.
 

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