Einstein Summation -- from Wolfram MathWorld

Einstein summation is a notational convention for simplifying expressions including summations of vectors, matrices, and general tensors. There are essentially three rules of Einstein summation notation, namely: 1. Repeated indices are implicitly summed over. 2. Each index can appear at most twice in any term. 3. Each term must contain identical non-repeated indices. The first item on the above list can be employed to greatly simplify and shorten equations involving tensors. For example,...