## Friday, December 20, 2002

### Experience Mathematics #23 -- Ambigrams (by Punya Mishra)

Symmetry is important in mathematics and in art. Today we will look at a special kind of wordplay based on ideas of symmetry and figure and ground. Consider the word below:

Can you read it? Now turn the page you are holding upside down and try reading it that way. The word stays the same. This image/word has rotational symmetry—essentially, it stays the same when rotated 1800.

Such visual wordplays are called ambigrams. The word “ambigram” was first coined by the cognitive scientist Douglas Hofstadter. Here is an ambigram of the word ambigram itself.

Ambigrams can be of many different kinds. For instance consider the word “logical” below.

This word has reflection symmetry i.e. it will read the same even when reflected in a mirror.

Some ambigrams are not about symmetry as much as they are about reading words in multiple ways. Here is one titled “good-evil” Can you see both words? Look carefully. This is similar to figure-ground paintings by M. C. Escher.

Creating ambigrams is great fun. Why don’t you try creating some yourself? If you want to see more examples of such wordplay you can search on Google or go to my wordplay gallery: http://punya.educ.msu.edu

This guest column has been written by Professor Punya Mishra, College of Education, Michigan State University, USA. You can email him at punya@msu.edu

## Friday, December 13, 2002

### Experience Mathematics # 22: The mobius strip

Take a long, thin strip of paper, give it a half twist, and paste the two ends together. What you get is a mobius strip (see picture).

Compare the mobius strip with the cylinder, which you get if you don’t give a half twist. A mobius strip has only one surface. Can you see why? Draw a line along the edge and keep going on. Eventually, you will arrive at the starting point. A cylinder, on the other hand, has an inside and an outside surface. The artist Escher portrayed this idea in Mobius Strip II (woodcut, 1963) (see picture).

If you cut the paper cylinder in half, you will get two cylinders. However, if you cut the mobius strip, you will get something very similar to a mobius strip. How many half-twists does the cut mobius strip have?

The mobius strip is one of the many surfaces that appear in Topology, a branch of mathematics. Topologists have been described as the mathematicians who cannot tell the difference between a coffee mug and a doughnut. This is because a mug can be “continuously deformed” to become a doughnut. A doughnut (which is a tyre tube, topologically speaking) cannot be transformed into a sphere. So, according to topologists, the tyre tube is not the same as a sphere, but a coffee mug is homotopic to a doughnut.

Pic credits: Both were stolen off the web, I don't know from where.

## Thursday, December 05, 2002

### Experience Mathematics # 21 -Euclid's fifth axiom

Euclid’s fifth axiom says that given a line $l$ and a point $P$ not on the line, there is exactly one line parallel to $l$ passing through the point $P$. For centuries people thought that Euclid’s fifth axiom was “obvious”. But some mathematicians did not find it obvious.

Finally, Reimann and Lobachevsky, both modified the axiom and tried to derive a new geometry.

Reimann began with the axiom: Given a line $l$ and a point $P$ not on the line, there is no line parallel to $l$ passing through the point $P$. Reimann derived many geometrical theorems that are applicable on the surface of a sphere. For example, he showed that the sum of angles of a triangle is always greater than 180 degrees. Try drawing a triangle on a sphere and see why this has to be true.

Similarly, if we take a hyperbola ($y=1/x$) and rotate it around the y-axis, then we obtain a surface where Lobachevsky’s geometry holds. Lobachevsky’s geometry contains the axiom: Given a line $l$ and a point $P$ not on the line, there is more than one line parallel to $l$ passing through the point $P$. In this geometry, the sum of angles of a triangle is always less than $180$ degrees.

There is a property of the surface (known as curvature) that determines the geometry. Only surfaces with curvature zero follow the Euclidean geometry. Another example of a surface is that of a saddle (of a horse). Can you tell which geometry is applicable on this surface?
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