To be used in future
 Appendix A: Useful Mathematics Up Appendix A: Useful Mathematics Section A.2: Coordinate systems 

A.1 Vector algebra

We introduce two useful objects, the Kronecker delta↓ and the Levi-Civita tensor↓ With these objects and the Einstein summation convention↓ (in its naive form: any repeated index is summed over, rather than the one used in general relativity) we find We can now write the scalar triple product↓ very elegantly and read of its symmetry under cyclic permutations of the three vectors.
The triple vector product↓ also falls quite easily If the vectors commute, we get the standard result
 Appendix A: Useful Mathematics Up Appendix A: Useful Mathematics Section A.2: Coordinate systems