 For example, in the homogeneous model for Type IX space, a signal is propagated in a direction in which for a long era, scales change by a law close to ~ "t".The square of the distance element in this direction is "dl"2 = "t"2formula_165, and the respective element of the four-dimensional interval is "ds"2 = "dt"2 − "t"2formula_165.The substitution "t" = "е"η puts this in the form
and for the signal propagation one has equation of the type again.