Let $\Gamma_n=Sp_{2n}(\mathbb{Z})$ and write $\gamma=\left(\matrix{A & B \\ C & D}\right)\in\Gamma_n$ for its block decomposition. Further let $\Gamma_n^0$ be the subset consisting of symplectic matrices such that $C=0$. Let $\mathcal{H}_n=\{Z\in\mathcal{M}_{n\times n}(\mathbb{C}): Z^t=Z, Y>0\}$, where $Z=X+iY$, be the usual Siegel upper half plane. We define $j(\gamma,Z)=\det(C_\gamma Z+D_\gamma)$.
The non-holomorphic Eisenstein series $E_n^{k,s}$ is defined as $$E_n^{k,s}(Z)=\det(Y)^s\sum_{\Gamma_n^0\backslash\Gamma_n}j(\gamma,Z)^{-k}|j(\gamma,Z)|^{-2s}$$ for $k\in\mathbb{N}$ and $s\in\mathbb{C}$. In order to prove its absolute convergence, we need to estimate $$\sum_{\Gamma_n^0\backslash\Gamma_n}|j(\gamma,Z)|^{\alpha}$$ where $\alpha\in\mathbb{R}$. I know that there are explicit conditions on $\alpha(k,s,n)$ so that the series is convergent, but I could not find any reference for a proof.
If $n=1$, this series turns out to be $\le c_0\sum_{c,d\in\mathbb{Z}}^{(c,d)\not=(0,0)}|cz+d|^{\alpha}$ for some constant $c_0$, and I have seen (again no reference) that this is $\le c_1 y^{\alpha+1}$ on the usual fundamental domain for $\Gamma_1\backslash \mathcal{H}_1$ (which I am happy to believe, as it looks like taking that sum is more or less like integrating over one variable, therefore raising the exponent by one).
Is something similar happening in general $n\ge 1$, maybe like $$\sum_{\Gamma_n^0\backslash\Gamma_n}|j(\gamma,Z)|^{\alpha}\le c_0 \det(Y)^{\alpha+n}$$ on a fundamental domain for $\Gamma_n\backslash \mathcal{H}_n$ chosen so that $Y\ge c_1 I$ (like Klingen explains, using the Minkowski reduced set)? Can you suggest any reference on the topic?
