I'm collection information for my upcoming project regarding eta-products and theta-series. I have the Günter Köhler's book,
http://www.amazon.com/Products-Identities-Springer-Monographs-Mathematics/dp/3642161510
and read this research paper, as well
http://www.kurims.kyoto-u.ac.jp/~kyodo/kokyuroku/contents/pdf/1617-14.pdf
My question here why people are interested in this kind of expression?
Many thanks in advance.