Hi, I will try to help you out :)
Lets consider an n-dimensional Euclidean space [tex]E_{n}[/tex] and by means of abstraction we develop the algebra of general affine tensors.
An orthonormal system [tex]e_{j}[/tex] in [tex]E_{n}[/tex] consists of n mutually orthogonal unit vectors. Any orthonormal system [tex]{\bar{e}_{j}}[/tex] may be obtained from the first by means of the linear transformation
[tex]\bar{e}_{j} = \sum^{n}_{h= 1} a_{ j, h} e_{h} (j = 1, ..., n)[/tex]
provided that the coefficients [tex]a_{jh}[/tex] satisfy the orthogonality condition
[tex]\delta _{j,k} = \sum^{n}_{h=1} a_{j,h} a_{k, h} (j, k = 1, ...,n),[/tex]
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