Question: What are the $q$ power series with $\pm1$ coefficients?
This question arose when I saw this:
$\ldots$ Thus, we may write$$f(-q^{1/5})=\sum\limits_{n=-\infty}^{\infty}(-1)^nq^{n(3n+1)/10}=J_1+q^{2/5}J_2+q^{1/5}J_3\tag1$$ Where $J_1,J_2,J_3$ are power series in $q$ with coefficients $\pm1$.
I'm not too sure what they meant by "power series in $q$ with coefficients $\pm1$." I tried searching up "power series in $q$" on the Internet, and nothing popped up.