\begin{table}%t1
%\centering
\par
  \caption{\label{tb:summary}Summary of the adopted main quantities.}
\begin{tabular}{lll}
  \hline\hline
  Data set & $\vec{d}\;\in\; \mathbb{R}^{n}$, &~ where ~ $n$ is the number of data points\\
  Training vectors & $\vec{t}_i \;\in\; \mathbb{R}^n$ &~ with ~ $i=1,...,M$\\
  Reference vector/model & $\bar{\vec{t}}\;\in\; \mathbb{R}^{n}$ & ~ e.g. ~ $\bar{\vec{t}}=\langle \vec{t}\rangle$\\
  %%$W:\mathbb{R}^n \rightarrow \mathbb{R}^n$
  Principal components & $\vec{w}_i \;\in\;\mathbb{R}^n$ & ~ with ~ $i=1,...,n$\\
  Feature vector & $\vec{\tau} = \vec{W}^{\rm T}\vec{t} \;\in\; \mathbb{R}^n$ & ~ where ~ $\vec{W}=\left(\vec{w}_1,\vec{w}_2, ..., \vec{w_n} \right)$\\
\hline
\end{tabular}
\end{table}