Important constants and tensor operations

In this section, some important rules, conventions, and constants are summarised. 

The Kronecker delta Sij represents a tensor with invariant components that do not change in any coordinate rotation. It is defined as 

Written in the component notation, the Kronecker delta is nothing but the unit tensor  

In a tensor of second order, this means exchanging the rows and columns. The trace of a tensor of second order is the sum of its diagonal elements  

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