Abstract
A theoretical attempt is made to describe local and propagating-wave disturbances with the method of complex characteristics and to examine whether the so-called absolute instability can occur in three-dimensional boundary layers whose basic state and stability properties vary in a specific direction of space. With a complex dispersion relation including one space variable, zeros of the complex group velocity are found not to produce such a drastic phenomenon as the absolute instability predicted in the parallel-flow problems studied so far. This is because the group velocity in the neighborhood of a zero varies in proportion to the square root of the distance from the zero.
