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An \m×\n A matrix is a <em>line matrix</em> if \m=1,
or <em>column matrix</em> if \n=1. A is a <em>square matrix</em> if
\m=\n.

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IN a matrix A=(\a<sub>\i\j</sub>), the coefficients \a<sub>11</sub>,
\a<sub>22</sub>, \a<sub>33</sub> ... are <em>diagonal</em> coefficients.
The matrix A is a <em>diagonal</em> matrix if all its non-diagonal coefficients are zero.

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The matrix A=(\a<sub>\i\j</sub>) is <em>upper-triangular</em>
(resp. <em>lower-triangular</em>) if all its coefficients below (resp. above)
the diagonal are zero, that is, \a<sub>\i\j</sub>&nbsp;=&nbsp;0 if \i &gt; \j
(resp. if \i &lt; \j).

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The <em>trace</em> of a matrix A=(\a<sub>\i\j</sub>) of dimension \m&times;\n
is the sum of diagonal coefficients:
 <div class="wimscenter">
  trace(A) = \(\sum_{i=1}^{\min(m,n)}a_{ii})
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<b>Remark</b>. Although the definitions of diagonal, trangular and trace are
valid for matrices of arbitrary dimensions, in general they are interesting
only for square matrices.