The gradient vector operator is defined as .

The Laplacian scalar operator is defined as .

The four partial-differential operators, , form a covariant vector, called the covariant gradient operator

(2.18) |

Also, the contravariant gradient is defined as (notice the minus sign)

(2.19) |

Finally, the d'Alembertian is defined as

(2.20) |

Sometimes the symbol is used for the d'Alembertian.

2004-03-18