Referring to this article:
https://i.stack.imgur.com/sfQ1C.png
and
https://i.stack.imgur.com/LelKb.png
How is it that they get from equation 2 to equation 3?
Whenever I do it, I get only cosine terms, no sine terms and can't cancel the imaginary ones like they do.
This is taking into account Euler's formula and even/odd properties of the trig functions (i.e. m terms combine with their -m counterparts)
Is there some spherical harmonics property that I'm missing out?