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Given the symplectic group $\mathrm{Sp}(2,\mathbb{R})$, represented as real $2\times 2$ matrices, I would like to compute the geodesic from the identity matrix $1\!\!1$ to the group element \begin{align} \left( \begin{array}{cc} \omega & 0\\ 0 & \frac{1}{\omega} \end{array}\right)\,, \end{align} where I define my right-invariant metric on the Lie algebra, such that the following the following basis of the Lie algebra is orthonormal: \begin{align} K_1=\left( \begin{array}{cc} 1 & 0\\ 0 & -1 \end{array}\right)\,,\quad K_2=\left( \begin{array}{cc} 0 & 1\\ -1 & 0 \end{array}\right)\,,\quad K_3=\left( \begin{array}{cc} 0 & 1\\ 1 & 0 \end{array}\right)\,. \end{align} I believe that the geodesic should be given by just \begin{align} \gamma(t)=\exp\left(t\ln{(\omega)}K_1\right)\,, \end{align} with $t\in[0,1]$.

Is there a direct way to see from the metric that this is a geodesic? Also, due to the right-invariance of the metric, we should have $3$ Killing vector fields that may help to find the geodesic?

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  • $\begingroup$ Why the title does not match the question? the title evokes $\mathrm{Sp}(2N,\mathbf{R})$, in the question $N=1$ which is just $\mathrm{SL}_2(\mathbf{R})$. $\endgroup$
    – YCor
    Commented May 23, 2017 at 22:40
  • $\begingroup$ @YCor: yes, that's true - I was originally interested in Sp(2N,R), but decided to focus on Sp(2,R) first. $\endgroup$
    – LFH
    Commented May 23, 2017 at 22:46
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    $\begingroup$ This is in Helgason, including the fact that there is no such geodesic if $-1\ne\omega<0$: problems II.A.5 and II.B.1, with solutions at the end of the book. $\endgroup$ Commented May 23, 2017 at 23:46
  • $\begingroup$ @FrancoisZiegler: This is helpful. However, in Helgason they use a pseudo-Riemannian metric, while I really use a metric where orthonormal means positive definite. In particular, my metric is NOT bi-invariant (only right-invariant by construction while left-invariance only holds for a subgroup). $\endgroup$
    – LFH
    Commented May 25, 2017 at 0:06
  • $\begingroup$ @LFH You may find my answer and comments about it to mathoverflow.net/questions/108280/… useful. The problem you are asking about is not entirely trivial. $\endgroup$ Commented Jul 8, 2017 at 19:47

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