Consider the Cauchy problem [GRAPHICS] where f : R --> R C-infinity, f
(0) = 0. After treatment of the local existence problem, we show the b
low up of the solution of the equation (1) under the following assumpt
ions. Let alpha greater-than 0 be real, and such that [GRAPHICS] where
P = 1 - partial derivative(2)/partial derivative x(2), F'(s) = f(s),
and nu(0) is given by u(t)(x, 0) = (upsilon(0)(X))(x). Then we focus o
n various perturbations of the equation. We also study the vectorial c
ase in the same way, and finally we give some examples.