Einstein notation

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In mathematics, the Einstein notation or Einstein summation convention is a convention useful when dealing with coordinate equations: when an index variable appears twice in a single term on the same side of an equation, we are summing over all its possible values.

For example, the dot product of two vectors a and b can be written:

The i is known as a dummy suffix since the result is not dependent on it, thus we could also write, for example

The Kronecker delta and the Levi-Civita symbol are frequently used in Einstein notation.

For example, the cross product of two vectors can be written: