## Linear Maps between vector spaces: Einstein Summation Convention

Einstein summation convention is a notational convention in Mathematics that is commonly used in the applications of linear algebra in continuum mechanics. The purpose is to achieve notational brevity. According to Einstein summation convention, when an index appears twice in a single term it implies summation of that term over all the values of the index which are almost always the values of since the underlying space is . For example, if is a basis set for , and , then applying Einstein summation convention implies the following equality:

The following equalities relating the Kronecker delta, the alternator, and the vectors of the basis set are very useful when dealing with continuum mechanics and with Einstein summation convention:

These, and using Einstein summation convention can be used to show the following identity:

Notice that the expression is actually summed over the values of to . However, is 0 except when and therefore, we are left with only one component .

Similarly, if ,

The cross product can be simplified using the alternator and the Einstein summation convention as follows. If , then the component of the vector has the form:

For example, the convention can also be used for the following component forms. Let , and then:

I think there is a small mistake:

Instead of ei.Mej=ei.Mkjek,it should be ei.Mej=ei.Mjkek

This is not a mistake. This is by definition:

Me_j = M_kj e_k

Basically, the vector Me_j has three components M_1j, M_2j, and M_3j in the basis set B={e_1, e_2, e_3}