You may also like

problem icon


There are three tables in a room with blocks of chocolate on each. Where would be the best place for each child in the class to sit if they came in one at a time?

problem icon


At the corner of the cube circular arcs are drawn and the area enclosed shaded. What fraction of the surface area of the cube is shaded? Try working out the answer without recourse to pencil and paper.

problem icon

Do Unto Caesar

At the beginning of the night three poker players; Alan, Bernie and Craig had money in the ratios 7 : 6 : 5. At the end of the night the ratio was 6 : 5 : 4. One of them won $1 200. What were the assets of the players at the beginning of the evening?

The Cantor Set

Stage: 3 Challenge Level: Challenge Level:2 Challenge Level:2

Take a line segment of length 1. We'll call it $C_1$.

Now remove the middle third. Call what's left $C_2$.

Now remove the middle third of each line segment in $C_2$. Call what's left $C_3$.

We can keep doing this, at each stage removing the middle third of each of the line segments in $C_n$ to form $C_{n+1}$.
Construction of cantor set

Draw pictures of $C_4$ and $C_5$.

If we suppose that the end points of $C_1$ are 0 and 1, then we can mark on the end points of the line segments for the later $C_n$ too. For example, $C_2$ has end points $0$, $\frac{1}{3}$, $\frac{2}{3}$ and $1$ as shown below.
Cantor sets with labels

Draw $C_3$ and label the end points, and label the end points on your pictures of $C_4$ and $C_5$.

We can keep removing middle thirds infinitely many times. The set of points left having done it infinitely many times is called the Cantor set.

Which of the following points are in the Cantor set?

$\frac{1}{3}$, $\frac{4}{9}$, $\frac{3}{81}$, $\frac{4}{81}$.

Explain how you decided which belong and which don't.


Full screen version

If you can see this message Flash may not be working in your browser
Please see to enable it.

See also the problem Smaller and Smaller.