Hello, in a paper http://www-library.desy.de/preparch/conf/theo-ws/workshop2004/data/Chatillon.pdf .(adsbygoogle = window.adsbygoogle || []).push({});

says that.

1) In 4d, it is a total derivative, then does not contribute to the equations of motion

A total derivative respect to time does not contribute to equation of motion???, ¿or other parameter??

2) Only [tex]R^2[/tex] order combination giving equations of motion

with no derivatives of higher order than two and divergence free, like the Einstein tensor.

¿why einstein tensor gives equation of motion of second derivative of time?...I think that varying the action [tex] \int \sqrt{g_{uv}dx^u dx^v} [/tex] i get the equation of motion, but i don't understand but the einstein tensor too...

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# Einstein gauss bonnet gravity

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