Can a Continuous Curve Only Intersect a Differentiable Curve at the Origin?

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Discussion Overview

The discussion revolves around a conjecture regarding the intersection of a continuous curve with a differentiable curve at the origin. Participants explore whether it is possible for a continuous function to intersect a given differentiable function only at the origin, while also being non-constant.

Discussion Character

  • Exploratory
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant conjectures that for a differentiable function $f:[0,1]\to\mathbb R^2$ with $f(0)=(0,0)$, there exists a continuous function $g:[0,1]\to\mathbb R^2$ that intersects $f$ only at the origin and is not constant.
  • Another participant questions the validity of defining a function $g(x)= f(x)+(x^2, x^2)$, suggesting that this could lead to intersections at points other than the origin.
  • A subsequent post reiterates the concern about the definition of $g$, emphasizing that it might yield $g(x_1)=f(x_1)+(x_1^2,x_1^2)=f(x_2)\neq (0,0)$ for some $x_1$ and $x_2$.
  • A different participant provides an example function $f(t)=t^3 \sin (1/t) e^{i\pi/t}$, noting that it is differentiable and expressing skepticism about the possibility of a function $g$ avoiding intersections with $f$ other than at the origin.

Areas of Agreement / Disagreement

Participants express differing views on the conjecture, with some questioning the proposed definitions of $g$ and others providing examples that challenge the feasibility of the conjecture. No consensus is reached regarding the validity of the conjecture or the proposed functions.

Contextual Notes

The discussion highlights potential limitations in the conjecture, including the dependence on the specific definitions of the functions involved and the behavior of differentiable functions near the origin. Unresolved mathematical steps and assumptions are present in the arguments.

caffeinemachine
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Hello MHB,

I have the following conjecture which I cannot seem to settle either way:

Let $f:[0,1]\to\mathbb R^2$ be a differentiable function such that $f(0)=(0,0)$.
Then there exists a continuous function $g:[0,1]\to\mathbb R^2$ such that:
1) $g(0)=(0,0)$
2) $g([0,1])\cap f([0,1])=\{(0,0)\}$.
3) $g$ is not a constant function. (Forgot to add this earlier.)

Basically what I am trying to prove is that if we have a differentiable curve in $\mathbb R^2$ whic passes through origin then we can find a continuous curve in $\mathbb R^2$ which intersects the gives curve only at origin.
 
Last edited:
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What's wrong with defining, say, g(x)= f(x)+ (x2, x2)?
 
HallsofIvy said:
What's wrong with defining, say, g(x)= f(x)+ (x2, x2)?
It might so happen that $g(x_1)=f(x_1)+(x_1^2,x_1^2)=f(x_2)\neq (0,0)$.
 
[From my phone by Tapatalk, so this has to be short. ]

Suppose $ f (t)=t^3 \sin (1/t) e^{i\pi/t} $, with $ f (0) =0$. This is differentiable, and I don't see how a function $ g $ can get away from the origin without crossing the graph of $ f $.
 
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