# 측지선

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## 개요

• n차원 다양체 M의 coordinate chart 에서 $$\alpha(t)=(\alpha_1(t),\alpha_2(t),\cdots, \alpha_n(t))$$ 로 표현되는 곡선이 측지선이 될 조건은 크리스토펠 기호를 사용하여 다음 미분방정식으로 쓸 수 있다

$\frac{d^2\alpha_k }{dt^2} + \sum_{i,j}\Gamma^{k}_{~i j }\frac{d\alpha_i }{dt}\frac{d\alpha_j }{dt} = 0,\quad k=1,2,\cdots, n$
또는$\ddot{\alpha_k } + \sum_{i,j}\Gamma^{k}_{~i j }\dot{\alpha_i}\dot{\alpha_j }= 0,\quad k=1,2,\cdots, n$

## 곡면의 측지선

• 곡선 ($$(x(t),y(t))$$ 가 다음의 미분방정식을 만족해야 한다$x''(t)+\Gamma _{1,1}{}^1 x'(t)^2+\Gamma _{1,2}{}^1 x'(t) y'(t)+\Gamma _{2,1}{}^1 x'(t) y'(t)+\Gamma _{2,2}{}^1 y'(t)^2=0$$y''(t)+\Gamma _{1,1}{}^2 x'(t)^2+\Gamma _{1,2}{}^2 x'(t) y'(t)+\Gamma _{2,1}{}^2 x'(t) y'(t)+\Gamma _{2,2}{}^2 y'(t)^2=0$

## 관련논문

• Christian Lange, On metrics on 2-orbifolds all of whose geodesics are closed, arXiv:1603.08455[math.DG], March 28 2016, http://arxiv.org/abs/1603.08455v1
• Radeschi, Marco, and Burkhard Wilking. “On the Berger Conjecture for Manifolds All of Whose Geodesics Are Closed.” arXiv:1511.07852 [math], November 24, 2015. http://arxiv.org/abs/1511.07852.
• Erlandsson, Viveka, and Juan Souto. “Counting Curves in Hyperbolic Surfaces.” arXiv:1508.02265 [math], August 10, 2015. http://arxiv.org/abs/1508.02265.
• Kennard, Lee, and Jordan Rainone. “Characterizations of the Round Two-Dimensional Sphere in Terms of Closed Geodesics.” arXiv:1507.00414 [math], July 1, 2015. http://arxiv.org/abs/1507.00414.
• Sapir, Jenya. ‘Lower Bound on the Number of Non-Simple Closed Geodesics on Surfaces’. arXiv:1505.06805 [math], 26 May 2015. http://arxiv.org/abs/1505.06805.