\begin{equation}\label{delta-l}  
D_{l}=\sum_{I_{l}\subseteq \mathbf{n}}  
D(t_1,\dots,t_n)\Bigr}  
_{t_i=\left\{\begin{smallmatrix}  
0,& \text{if }i\in I_{l}\quad\\% \quad added for centering  
1,& \text{otherwise}\end{smallmatrix}\right.\;,\;\; i=1,\dots,n}.  
\end{equation}