Holomorphic eta quotients of weight 1/2

We give a short proof of Zagier’s conjecture / Mersmann’s theorem which states that each holomorphic eta quotient of weight 1/2 is an integral rescaling of some eta quotient from Zagier’s list of fourteen primitive holomorphic eta quotients. In particular, given any holomorphic eta quotient $f$ of weight 1/2, this result enables us to provide a closed-form expression for the coefficient of qn in the $q$-series expansion of $f$, for all $n$. We also demonstrate another application of the above theorem in extending the levels of the simple (resp. irreducible) holomorphic eta quotients.