We always may assume the following "natural conditions " to the used functions:

**m**,**D**and**e**are continuous, in most cases they are also differentable.**m**> 0. Often**m**is a norm, e.g. l_{p}- The value of
**m**does not depend on the order of coordinates; i.e.:

**m**( ..x_{i-1},x_{i}, x_{i+1},..., x_{j-1},x_{j}, x_{j+1},.. ) =**m**( ..x_{i-1},x_{j}, x_{i+1},..., x_{j-1},x_{i}, x_{j+1},.. ), -
**D**( p_{1},p_{2},p_{3}) > 0 for each positive orientated triangle p_{1},p_{2},p_{3}. - In most cases
**D**has a kind of "monotony":

*Among all triangles with given base AB the isoscele has extremal value (valued by*.**D**) -
**e**( p_{1},p_{2}) > 0, equality if and only if p_{1}= p_{2}