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© 1999-2010 by Francis Edward Su

From the Fun Fact files, here is a Fun Fact at the Easy level:

# Fractional Dimensions

A point has dimension 0, a line has dimension 1, and a plane has dimension 2. But did you know that some objects can be regarded to have "fractional" dimension?

You can think of dimension of an object X as the amount of information necessary to specify the position of a point in X. For instance, a block of wood is 3-dimensional because you need three coordinates to specify any point inside.

The standard Cantor set has fractional dimension! Why? Well it is at most 1-dimensional, because one coordinate would certainly specify where a point is. However, you can get away with "less", because the object is self-similar. At each stage, you only need to specify which 2 out of 3 segments a point is in. Mathematicians have developed a notion of "dimension" which for the standard Cantor set works out to be:

ln(2)/ln(3) = 0.6309...
Most other "fractals" have fractional dimension; for instance a curve whose boundary is very, very intricate can be expected to have dimension between 1 and 2 but closer to 2. This concept has been applied in other sciences to describe structures that appear to have some self-similarity, such as the coast of England or gaseous nebulae in interstellar space.

Presentation Suggestions:
It makes sense to do this Fun Fact after doing the one on the standard Cantor set.

The Math Behind the Fact:
Actually, the notion of "dimension" can be extended to crazy sets in many different ways. One notion is box dimension, and another is Hausdorff dimension. These notions agree for the standard Cantor set and many other sets.

Su, Francis E., et al. "Fractional Dimensions." Math Fun Facts. <http://www.math.hmc.edu/funfacts>.

References:
K. Falconer, Fractal Geometry.

Keywords:    standard Cantor set
Subjects:    geometry
Level:    Easy
Fun Fact suggested by:   Francis Su
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