3 Trace of a matrix

The trace of a matrix is simply the sum of its diagonal elements. Thus the trace of the matrix

$$\begin{eqnarray*}\left( \begin{array}{rrr} 1 & 2 & 3\ 4 & 5 & 6 \ 7 & 8 & 9 \end{array} \right) = 1 + 5 + 9 = 15 \end{eqnarray*}$$