i have been reading about 'Selberg Trace formula'(adsbygoogle = window.adsbygoogle || []).push({});

i know what a Laplacian is but i do not know what is the author referring to when he talks about 'Closed Geodesic' i know what the Geodesic of a surface is

[tex] l(\gamma)=\int_\gamma \sqrt{ g(\dot\gamma(t),\dot\gamma(t)) }\,dt\ ,[/tex]

but i do not know what means 'closed' or why the geodesic of a torus would have the lenght (?) [tex] l_n =na [/tex] a=radius ??

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# What are the 'closed geodesic' ?

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