The Einstein notation is a convention of notation of mathematical expressions within the Ricci calculus and represents an index notation. This calculus is in the tensor analysis , the differential geometry , and particularly in theoretical physics used. The sum convention was introduced by Albert Einstein in 1916 . With it, the summation symbols are simply omitted to improve the overview and instead a total is made using double indices.
In matrix and tensor calculus, sums are often formed using indices. For example, the matrix product of two matrices and in components is:
Here the index is totaled from 1 to . If several matrix multiplications, scalar products or other sums occur in one calculation, this can quickly become confusing. With Einstein's summation convention, the calculation from above is then:
In the simplest case of the summation convention, the following applies: Duplicate indices within a product are used to sum up. In the theory of relativity, there is an additional rule: It is only added if the index occurs as both an upper (contravariant) and a lower ( covariant ) index.
Above all, the summation convention reduces the writing effort. Sometimes it helps to highlight existing relationships and symmetries that are not so easily recognizable in conventional sums.
Without considering the index position
The following examples are for matrices with entries and for matching vectors.