64 |
\end{equation} |
\end{equation} |

65 |
We often find that use |
We often find that use |

66 |
of nested $\sum$ symbols makes formulas cumbersome, and we use the more |
of nested $\sum$ symbols makes formulas cumbersome, and we use the more |

67 |
convenient Einstein summation convention\index{summation convention}. This |
compact Einstein summation convention\index{summation convention}. This |

68 |
drops the $\sum$ sign and assumes that a summation is performed over any repeated index. |
drops the $\sum$ sign and assumes that a summation is performed over any repeated index. |

69 |
For instance we write |
For instance we write |

70 |
\begin{eqnarray} |
\begin{eqnarray} |