Explicit equations of motion for axial perturbations in Horndeski

[Héloïse]

Hi everyone,

I am considering a specific class of scalar-tensor Horndeski theories. I have already obtained the equations of motion for the background metric and scalar field combining xTensor with xCoba using CTensors for their explicit evaluation.

I am now trying to apply a similar method to obtain the explicit equations of motion for the first-order and axial metric perturbation. However, I have many more terms in that case (around 2700 tensorial terms) and decomposing them in a list and evaluate each of them separately does not always work, as some terms take way too long, for example those containing 3 covariant derivatives of the metric perturbation (e.g. term 36 in my MWE).

Could you please help me to accelerate the explicit evaluation of those equations of motion? I would also appreciate to learn why terms like term 36 take that long within the formalism of CTensors, and I would of course be open to try another approach that is faster/more efficient.

Thank you in advance for your detailed help!

Best,

Héloïse