INTRODUCTION

During the last decade it has widely been recognized by physicists working in diverse areas that many of the structures common in their experiments possess a rather special kind of geometrical complexity. This awareness is largely due to the activity of Benoit Mandelbrot (1977, 1979, 1982, 1988), who called attention to the particular geometrical properties of such objects as the shore of continents, the branches of trees, or the surface of clouds. He coined the name fractal for these complex shapes to express that they can be characterized by a non-integer (fractal) dimensionality. With the development of research in this direction the list of examples of fractals has become very long, and includes structures from microscopic aggregates to the clusters of galaxies…