Einstein notation
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: