An explicit expansion formula for the powers of the Euler Product in terms of partition hook lengths

We discover an explicit expansion formula for the powers $s$ of the Euler Product (or Dedekind $\eta$-function) in terms of hook lengths of partitions, where the exponent $s$ is any complex number. Several classical formulas have been derived for certain integers $s$ by Euler, Jacobi, Klein, Fricke, Atkin, Winquist, Dyson and Macdonald. In particular, Macdonald obtained expansion formulas for the integer exponents $s$ for which there exists a semi-simple Lie algebra of dimension $s$. For the type $A_l^{(a)}$ he has expressed the $(t^2-1)$-st power of the Euler Product as a sum of weighted integer vectors of length $t$ for any integer $t$. Kostant has considered the general case for any positive integer $s$ and obtained further properties. ----- The present paper proposes a new approach. We convert the weighted vectors of length $t$ used by Macdonald in his identity for type $A_l^{(a)}$ to weighted partitions with free parameter $t$, so that a new identity on the latter combinatorial structures can be derived without any restrictions on $t$. The surprise is that the weighted partitions have a very simple form in terms of hook lengths of partitions. As applications of our formula, we find some new identities about hook lengths, including the "marked hook formula". We also improve a result due to Kostant. The proof of the Main Theorem is based on Macdonald's identity for $A_l^{(a)}$ and on the properties of a bijection between $t$-cores and integer vectors constructed by Garvan, Kim and Stanton.