Q4a: What is fractal dimension? How is it calculated?  
A4a: A common type of fractal dimension is the Hausdorff-Besicovich  
Dimension, but there are several different ways of computing fractal  
dimension.  
  
Roughly, fractal dimension can be calculated by taking the limit of the quo-  
tient of the log change in object size and the log change in measurement  
scale, as the measurement scale approaches zero. The differences come in  
what is exactly meant by "object size" and what is meant by "measurement  
scale" and how to get an average number out of many different parts of a  
geometrical object. Fractal dimensions quantify the static *geometry* of an  
object.  
  
For example, consider a straight line. Now blow up the line by a factor of  
two. The line is now twice as long as before. Log 2 / Log 2 = 1,  
corresponding to dimension 1. Consider a square. Now blow up the square  
by a factor of two. The square is now 4 times as large as before (i.e. 4  
original squares can be placed on the original square). Log 4 / log 2 = 2,  
corresponding to dimension 2 for the square. Consider a snowflake curve  
formed by repeatedly replacing ___ with _/\_, where each of the 4 new lines  
is 1/3 the length of the old line. Blowing up the snowflake curve by a factor  
of 3 results in a snowflake curve 4 times as large (one of the old snowflake  
curves can be placed on each of the 4 segments _/\_).  
Log 4 / log 3 = 1.261... Since the dimension 1.261 is larger than the  
dimension 1 of the lines making up the curve, the snowflake curve is a  
fractal.  
  
For more information on fractal dimension and scale, access via the WWW  
http://life.anu.edu.au/complex_systems/tutorial3.html .  
  
Fractal dimension references:  
  
[1] J. P. Eckmann and D. Ruelle, _Reviews of Modern Physics_ 57, 3  
(1985), pp. 617-656.  
  
[2] K. J. Falconer, _The Geometry of Fractal Sets_, Cambridge Univ.  
Press, 1985.  
  
[3] T. S. Parker and L. O. Chua, _Practical Numerical Algorithms for  
Chaotic Systems_, Springer Verlag, 1989.  
  
[4] H. Peitgen and D. Saupe, eds., _The Science of Fractal Images_,  
Springer-Verlag Inc., New York, 1988. ISBN 0-387-96608-0. This book  
contains many color and black and white photographs, high level math, and  
several pseudocoded algorithms.  
  
[5] G. Procaccia, _Physica D_ 9 (1983), pp. 189-208.  
  
[6] J. Theiler, _Physical Review A_ 41 (1990), pp. 3038-3051.  
  
References on how to estimate fractal dimension:  
  
1. S. Jaggi, D. A. Quattrochi and N. S. Lam, Implementation and  
operation of three fractal measurement algorithms for analysis of remote-  
sensing data., _Computers & Geosciences_ 19, 6 (July 1993), pp. 745-767.  
  
2. E. Peters, _Chaos and Order in the Capital Markets_, New York, 1991.  
ISBN 0-471-53372-6 Discusses methods of computing fractal dimension.   
Includes several short programs for nonlinear analysis.  
  
3. J. Theiler, Estimating Fractal Dimension, _Journal of the Optical Society  
of America A-Optics and Image Science_ 7, 6 (June 1990), pp. 1055-1073.  
  
There are some programs available to compute fractal dimension. They are  
listed in a section below (see "Fractal software").  
  
Q4b: What is topological dimension?  
A4b: Topological dimension is the "normal" idea of dimension; a point has  
topological dimension 0, a line has topological dimension 1, a surface has  
topological dimension 2, etc.  
  
For a rigorous definition:  
  
A set has topological dimension 0 if every point has arbitrarily small  
neighborhoods whose boundaries do not intersect the set.  
  
A set S has topological dimension k if each point in S has arbitrarily small  
neighborhoods whose boundaries meet S in a set of dimension k-1, and k is the  
least nonnegative integer for which this holds.