Editing Einstein notation (section)

===Application===

Einstein notation can be applied in slightly different ways. Typically, each index occurs once in an upper (superscript) and once in a lower (subscript) position in a term; however, the convention can be applied more generally to any repeated indices within a term.<ref name="wolfram">{{cite web |url=http://mathworld.wolfram.com/EinsteinSummation.html |title=Einstein Summation |access-date=13 April 2011 |publisher=Wolfram Mathworld }}</ref> When dealing with [[Covariance and contravariance of vectors|covariant and contravariant]] vectors, where the position of an index indicates the type of vector, the first case usually applies; a covariant vector can only be contracted with a contravariant vector, corresponding to summation of the products of coefficients. On the other hand, when there is a fixed coordinate basis (or when not considering coordinate vectors), one may choose to use only subscripts; see ''{{section link||Superscripts and subscripts versus only subscripts}}'' below.