I need to refind a reference on multigraded Hilbert series

Let $R$ be a Noetherian multigraded algebra $R:=\bigoplus_{j\in\mathbb{N}^m}{R_j}$ over $R_0=\mathbb{C}$. If $R$ is generated by $h$ homogeneous polynomials of multidegrees $\alpha_1,\dots,\alpha_h$, then its Hilbert series $HS_R(\mathbf{t})$ can be expressed as $$HS_R(\mathbf{t})=\frac{P(\mathbf{t})}{\prod_{\ell=1}^h{(1-\mathbf{t}^{\alpha_\ell})}},$$ where $P(\mathbf{t})$ is a polynomial with integer coefficients. Furthermore, if $R$ is freely generated, then $P(\mathbf{t})=1$. (here $\mathbb{t}^{\alpha}=t_1^{\alpha_1}\cdots t_n^{\alpha_n}$).