Let (M,g)(M,g) be a with ∇\nabla. A smooth curve γ:I→M\gamma:I\to M is a geodesic if ∇γ˙γ˙=0\nabla_{\dot\gamma}\dot\gamma=0 along II.

Existence and speed

Its speed ∥γ˙(t)∥g\|\dot\gamma(t)\|_g is constant. Given (x,v)∈TM(x,v)\in TM, there is a unique geodesic with γ(0)=x\gamma(0)=x and γ˙(0)=v\dot\gamma(0)=v on its maximal interval of existence. If ∥v∥g=1\|v\|_g=1, it is unit speed.

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