§ 2. **Mathematical auxiliary formulas.**

The time differentiation for fixed space points, is represented by . The temporal change of a surface integral, extended over a surface whose points are moving with velocity , namely

defines another kind of time differentiation of a vector

(1) |

Furthermore, the derivative (with respect to time) which is related to moving points, is

(2) |

This is connected with the temporal change of the volume integral of a vector, by the relations

(2a) |

From (2) and (2a) it follows

(3) |

Accordingly it is given for the scalars:

(3a) |

From (1) and (3) it finally follows, with respect to the general rule

,

the relation

(4) | . |

Since the time differentiation introduced in (2) follows the ordinary calculation rules, we have with respect to (2a)

From this equation, together with the ones following from (4) and (2a)