2.1.5 Solution along characteristic curves
For a quasilinear PDE

(67) 
a curve
with

(68) 
or sometimes the corresponding map
is called
characteristic curve
or
characteristic.
For the linear advection equation (63) these curves have the general form

(69) 
Along
we get for a solution
of Eq. (63)

(70) 
The solution
is
constant along the
characteristic
.