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Let $M$ be a compact manifold, and let $X$ be a vector field. Then its flow, $\varphi$, generates a $1$-parameter group of diffeomorphisms $\{\varphi_t\}$. For each $t$, we have a map $\varphi_t:M\to M$. Thus, for $p\in M$, the differential $\mathrm d(\varphi_t)_p:T_pM\to T_{\varphi_t(p)}M$ is defined. Is there a general formula for $\mathrm d (\varphi_t)_p(v),v\in T_pM$?

I am particularly interested in the case when $X=\mathrm{grad}\,f$, where $f$ is a smooth function and the gradient is determined using a Riemannian metric $g$ on $M$ and when $p$ is a critical point of $f$. Then $X$ vanishes at $p$ and the flow has $p$ as a fixed point, so $\mathrm d(\varphi_t)_p:T_pM\to T_pM$ is an endomorphism.

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  • $\begingroup$ What does "a general formula for this" even mean? $\endgroup$
    – user98602
    Commented Aug 8, 2016 at 22:15
  • $\begingroup$ @MikeMiller I updated the post. $\endgroup$
    – Ryan Unger
    Commented Aug 8, 2016 at 22:20
  • $\begingroup$ I understand what you're looking for a "general formula" for. I don't understand what form you think this could possibly take. That's what I want clarification on. $\endgroup$
    – user98602
    Commented Aug 8, 2016 at 22:21
  • $\begingroup$ @MikeMiller I have a specific example in mind, but the way it's phrased in the book makes me think it's perhaps a special case of a well-known, more general, result. (Page 38 in Brendle, Ricci Flow and the Sphere Theorem.) If there's no general result, I'll ask a specific question. $\endgroup$
    – Ryan Unger
    Commented Aug 8, 2016 at 22:23

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I'm not sure if this answers your question, but it may be a step in the right direction. There is a nice equation called the first variation equation for the flow giving you a differential equation for the object you're interested in if you can localize in $\Bbb R^n$: $$\frac d{dt} d(\varphi_t)(p) = dX(\varphi_t(p))d(\varphi_t)(p), \quad d(\varphi_0)(p) = \text{id}.$$ See also this SE question. So perhaps you know enough to get information by integrating this differential equation from $0$ to $t$.

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  • $\begingroup$ Hey Ted. I was hoping that there was an applicable general result, but the specific result I'm interested in proving can be found here. I'd appreciate your thoughts on it. (Perhaps this equation applies.) $\endgroup$
    – Ryan Unger
    Commented Aug 11, 2016 at 0:16

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