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### traceless part of stress tensor

Posted: **April 13 2019**

by **Angela Chen**

Hi all,

I have a dumb question that how was the traceless part of stress tensor (pic,pir,pig) defined in derivs code, I didn't find it in the CAMB notes...

Thanks!

Angela

### Re: traceless part of stress tensor

Posted: **April 15 2019**

by **Antony Lewis**

Just by direct expansion of the stress-energy tensor (Eq 24) where $\Pi = \sum_i \rho_i\pi_i$. You can read off the relevant equations from the multipole equations for $\ell=2$ (e.g. $I_2=\pi_\gamma$).

### Re: traceless part of stress tensor

Posted: **April 19 2019**

by **Angela Chen**

Thanks! another question, if I do the calculation in not no_nu_multipole for massless neutrino all the time and not using the approximation for large ktau modes, would it give me the right physics, just much slower?

### Re: traceless part of stress tensor

Posted: **April 19 2019**

by **Antony Lewis**

I should do, though you may need to also increase l_accuracy_boost.