Prove: The Frenet Formula for Torsion & Curvature

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SUMMARY

The discussion focuses on proving the relationship between curvature and torsion in a regular curve in \(\mathbb{R}^3\) with arc-length parametrization. Specifically, it establishes that if the torsion \(\tau(s) \neq 0\) and there exists a vector \(Y\) such that \(\langle \alpha', Y \rangle = A\), then \(\frac{k(s)}{\tau(s)} = B\) for some constant \(B\). The participants reference the Frenet formula, particularly \(n' = -k t - \tau b\), and explore implications of constant curvature-to-torsion ratios, leading to further properties of the Frenet frame.

PREREQUISITES
  • Understanding of Frenet-Serret formulas
  • Knowledge of curvature \(k(s)\) and torsion \(\tau(s)\) in differential geometry
  • Familiarity with inner product notation and vector calculus in \(\mathbb{R}^3\)
  • Concept of arc-length parametrization of curves
NEXT STEPS
  • Study the derivation and applications of the Frenet-Serret formulas
  • Explore the implications of constant curvature-to-torsion ratios in curves
  • Learn about the geometric interpretation of curvature and torsion
  • Investigate the properties of the Frenet frame and its applications in physics
USEFUL FOR

Mathematicians, physics students, and anyone studying differential geometry or the properties of curves in three-dimensional space will benefit from this discussion.

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Homework Statement



Suppose \alpha is a regular curve in \mathbb{R}^3 with arc-length parametrization such that the torsion \tau(s)\neq 0, and suppose that there is a vector Y\in \mathbb{R}^3 such that <\alpha',Y>=A for some constant A. Show that \frac{k(s)}{\tau(s)}=B for some constant B, where k(s) is the curvature of alpha.

The Attempt at a Solution



I think the Frenet formula in question that I can use is n'=-kt-\tau b, but I can't make it work.
 
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What about the converse? Suppose k/tau is constaint and tau is nonzero everywhere, show that there exists a nonzero vector Y such that <a', Y> is constant.
 
Ok, I'm pretty rusty at this so bear with me. Take k(s) nonzero (if it is zero, it's not even clear to me how to define the Frenet frame). Since you have arc-length parametrization, alpha'=t (the tangent). So <t,Y> is constant. Now you should be able to prove a bunch of stuff by differentiating <x,Y> where x is various vectors.

i) Show <n,Y>=0.
ii) Show <b,Y> is also constant.
iii) Differentiate <n,Y> and use your favorite Frenet formula.
 

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