Editing Square matrix (section)

===Trace===
The [[trace of a matrix|trace]], tr(''A'') of a square matrix ''A'' is the sum of its diagonal entries. While matrix multiplication is not commutative, the trace of the product of two matrices is independent of the order of the factors: 
<math display="block">\operatorname{tr}(AB) = \operatorname{tr}(BA).</math>
This is immediate from the definition of matrix multiplication:
<math display="block">\operatorname{tr}(AB) = \sum_{i=1}^m \sum_{j=1}^n A_{ij} B_{ji} = \operatorname{tr}(BA).</math>
Also, the trace of a matrix is equal to that of its transpose, i.e.,
<math display="block">\operatorname{tr}(A) = \operatorname{tr}(A^{\mathrm T}).</math>