What is the coefficient of $q^n$ in the Lambert Series Expansion $a^2(q).a(q^5)$. Here $a(q)$ is Borwein Theta Function.
I am using Ramanujan's Theta Functions Book by Shaun Cooper as a reference material and I am suppose to find the coefficient of $q^n$ in the lambert series for $a^2(q).a(q^5)$. But, I have no idea how to proceed. Any help will be greatly appreciated. Kindly provide me some hints or some resources that I can avail where I can find more about this.
Thanks in advance!