This post has moved to inclusion/exclusion, a blog of the American Mathematical Society.

# Nineteenth-Century Monsters

The nineteenth century was a time of change, when long-held assumptions about human society and the surrounding world were being questioned and overturned. It shouldn’t come as a surprise that it brought about an upheaval in the foundations of mathematics as well. It was an era of controversy and bitter contention. One of the main points of issue was *cardinality*.

The theory of sets and the idea of cardinality were first made rigorous by the mathematician Georg Cantor (1845 – 1918). At the time his work provoked widespread hostility, but posterity has vindicated him. As David Hilbert famously remarked:

No one shall expel us from the Paradise that Cantor has created.

Now his theory of sets forms part of most elementary education preparation programs.

Simply put, two sets are said to have the same cardinality if they can be paired off with one another without using anything more than once or leaving anything out. If a set has the same cardinality as the set

of natural numbers, then it is said to be *countable*. This means that there is a numbering scheme that eventually numbers every object in the set. I discussed this in my post on counting without numbers.

For instance, consider the set of rational numbers (numbers that can be written as fractions) on the line segment from 0 to 1 on the number line. Every number in this set can be written as *a*/*b*, where *a *≤ *b*. Consider the following list:

It should be clear that every rational number between 0 and 1 will eventually appear on this list. Of course, there are many repeats. For instance, the rational numbers 1/2, 2/4, 3/6, and so on are all equal to each other. So remove any unreduced fraction from the list. This deletes 2/4, and 2/6, and 3/6, and 4/6, and so on.

This lists every rational number exactly once. Now number off down the list. This shows that the set is countable.

On the other hand, the line segment from 0 to 1 is not countable. No matter how we try to pair the segment off with the natural numbers, there will always be infinitely many points of the segment left out. Intuitively, this is hardly surprising: the line segment is a continuum of points, and we wouldn’t expect that we could list them like we can the natural numbers. This idea of there being different grades of infinity was surprising to Cantor’s contemporaries. Even philosophers and theologians took note of it.

Given that the line segment is “more infinite” that the set of natural numbers, we might ask if there exists an uncountable set of points that *isn’t* a continuum. It turns out that there is: the Cantor ternary set, introduced by Georg Cantor in 1883. Take the line segment from 0 to 1 and remove the middle third, resulting in the line segments from 0 to 1/3 and from 2/3 to 1. Do the same for each of these segments, and so on, ad infinitum.

The resulting set is uncountable, contains no segments, and in fact has measure 0.

Cantor also showed that the line segment (which has topological dimension 1) has the same cardinality as the unit square (which has dimension 2). The notion that spaces of different dimension could be paired off in this way was so shocking that Cantor himself disbelieved the result for several years. One mathematician remarked:

It appears repugnant to common sense. The fact is that this is simply the conclusion of a type of reasoning which allows the intervention of idealistic fictions, wherein one lets them play the role of genuine quantities even though they are not even limits of representations of quantities. This is where the paradox resides.

Cantor’s ideas were savagely attacked by many of his fellow mathematicians; he was denounced as a “scientific charlatan” and a “corrupter of youth.” This exacerbated the bouts of mental illness he dealt with throughout his life, and he spent much of his time in sanatoriums.

Inspired by Cantor’s discovery, the Italian mathematician Guiseppe Peano set out to find a continuous mapping from the segment to the square that would cover the entire square. He did this in 1890 through an arithmetic description, and the curve was later interpreted geometrically as the limit of an iterative process. The result was the first known plane-filling curve. Another, better-known example was discovered by David Hilbert in 1891:

Although none of the iterative steps are self-intersecting, their limit contacts itself at every point. There is no *continuous* pairing of the line segment with the square. (Cantor’s mapping, while a true pairing, jumps around the square, whereas Hilbert’s mapping, while continuous, revisits each point multiple times.)

Another famous “pathological” object from the end of the nineteenth century is the Koch curve, named for its discoverer, the Swiss mathematician Helge von Koch (1870 – 1924). Beginning with a line segment, replace the middle third with the two legs of the equilateral triangle erected upon it. Repeat this process for each of the four segments that result, and so on, ad infinitum.

Each iterative step is a simple polygonal curve of dimension 1. But the limit of these steps—the Koch curve—is something else entirely. It is a *continuous* curve with an *infinite* length. For, if the length of the initial segment is 1 unit, then the length of the second step is 4/3 units, and that of third, 4/3 ⋅ 4/3 = 16/9 units, and so on. This sequence of lengths tends to infinity.

Each portion of the curve winds back and forth infinitely many times; in fact, each portion resembles the whole. More specifically, the Koch curve contains four copies of itself, each scaled down by a factor of one third. It is a continuous curve whose derivative exists at no point; this amounts to saying that at no point is there a single tangent line. The existence of such curves was profoundly disturbing to the mathematicians of the late nineteenth century; in 1893, the mathematician Charles Hermite wrote:

I turn away with fear and horror from this lamentable plague of functions with no derivatives.

If the above process is applied to each side of an equilateral triangle, then the figure known as the Koch snowflake results.

It is possible to compute the area of the snowflake. Taking its initial side length to be 1, its initial area is √3/4. Adding on the three triangles at the second step, the area becomes

At the third step, we add on 12 triangles, each of side length 1/9, so the area becomes

Continuing this pattern, we find that the area is

On the other hand, we’ve seen that the perimeter tends to infinity. So here we have a figure with a finite area and an infinite perimeter.

Plane-filling curves and the Koch snowflake are brought together in a beautiful way by a family of curves designed by Benoit Mandelbrot in the 1970s.

The limiting curve fills the Koch snowflake. At each stage the figure consists of a single open, simply connected polygonal curve. In principle this entire curve can be traced on paper without lifting one’s pencil or crossing a line, beginning at the lower left-hand point of the star and ending at the lower right-hand point. Filling in the polygonal curve produces a closed polygon. As Mandelbrot says:

This advanced teragon, shown as boundary between two fantastically intertwined domains serves better than any number of words to explain what plane-filling means.

The word *teragon*, coined by Mandelbrot, comes from the Greek roots *tera*, or “monster,” and *gon*, or “corner.”

*All the animations in this post were created by me using GeoGebra software.*

# Euclid Alone Has Looked on Beauty Bare

Euclid alone has looked on Beauty bare.

Let all who prate of Beauty hold their peace,

And lay them prone upon the earth and cease

To ponder on themselves, the while they stare

At nothing, intricately drawn nowhere

In shapes of shifting lineage; let geese

Gabble and hiss, but heroes seek release

From dusty bondage into luminous air.

O blinding hour, O holy, terrible day,

When first the shaft into his vision shone

Of light anatomized! Euclid alone

Has looked on Beauty bare. Fortunate they

Who, though once only and then but far away,

Have heard her massive sandal set on stone.

—Edna St. Vincent Millay

# GeoGebra Art

In the classes I teach I often use GeoGebra for demonstrations; I assign GeoGebra constructions as homework, and our senior math majors use it in their oral presentations. GeoGebra, if you don’t know, is a Java-based dynamic geometry program freely available on the web. GeoGebraTube has thousands of applets developed by members for use in the classroom.

Now, GeoGebra is used mainly by high school teachers, and most of the applets available online are developed with that in mind. But the emphasis on algebra and coordinates by state curricula means that the program’s possibilities are vastly curtailed in common practice.

In trying to explore the software’s capabilities, I’ve set myself the task of constructing some of the fractals described in Mandelbrot’s *Fractal Geometry of Nature*. Images of my first attempts are shown below. At this point I’m not going to go into what makes a fractal a fractal, contenting myself merely with the following quatrain, which comes from Jonathan Swift:

*So, Nat’ralists observe, a Flea,
Hath smaller Fleas that on him prey,
And these have smaller Fleas to bit ’em,
And so proceed ad infinitum.*

Click on an image to view the slideshow and read the captions. Interactive versions can be found by going to my GeoGebra profile page. I won’t expand on my cleverness in the succinct programming that produced these images, but some of them are extremely clever.

# The Quadrivium

Again, to start afresh, since of quantity one kind is viewed by itself, having no relation to anything else, as “even,” “odd,” “perfect,” and the like, and the other is relative to something else and is conceived of together with its relationship to another thing, like “double,” “greater,” “smaller,” “half,” “one and one-half times,” “one and one-third times,” and so forth, it is clear that two scientific methods will lay hold of and deal with the whole investigation of quantity; *arithmetic*, absolute quantity, and *music*, relative quantity.

And once more, inasmuch as part of “size” is in a state of rest and stability, and another part in motion and revolution, two other sciences in the same way will accurately treat of “size,” *geometry *the part that abides and is at rest, *astronomy *that which moves and revolves.

…In Plato’s *Republic*, when the interlocutor of Socrates appears to bring certain plausible reasons to bear upon the mathematical sciences, to show that they are useful to human life; arithmetic for reckoning, distributions, contributions, exchanges, and partnerships, geometry for sieges, the founding of cities and sanctuaries, and the partition of land, music for festivals, entertainment, and the worship of the gods, and the doctrine of the spheres, or astronomy, for farming, navigation and other undertakings, revealing beforehand the proper procedure and suitable season, Socrates, reproaching him, says: “You amuse me, because you seem to fear that these are useless studies that I recommend; but that is very difficult, nay, impossible. For the eye of the soul, blinded and buried by other pursuits, is rekindled and aroused again by these and these alone, and it is better that this be saved than thousands of bodily eyes, for by it alone is the truth of the universe beheld.”

–Nichomachus of Gerasa, *Introduction to Arithmetic*

# The Eccentric Circle

When you were in high school, you undoubtedly encountered conics at some point. The most familiar example is the parabola, introduced in algebra courses as the graph of the equation *y* = *x*^{2} or, more generally, *y* = *ax*^{2} + *bx *+ *c*. You may have learned how to find the *x*– and *y*– intercepts, the coordinates of the vertex, and so on. These are all useful skills–although I personally have never used them outside of a classroom–but they tell you very little about the parabola as a geometrical object. The other conic sections–the ellipse and the hyperbola–don’t lend themselves to a description of the form *y* = *f*(*x*), hence are usually not studied at all. This is a terrible misfortune, as they are part of the treasure that comes down to us from the ancient geometers, and figure largely in physical sciences such as optics, acoustics, and celestial mechanics.

The most famous way to describe conics is as slices of cones, hence the name. In plane geometry, however, it is more convenient to think of them in a different way. We start with a line *l* (the *directrix*) and a point *F* not on the line (the *focus*). We select an eccentricity *ε* > 0. The conic is the locus of all points *P* such that the ratio of *PF* to the perpendicular distance from *P* to *l* is *ε*. If *ε *< 1, the conic is an ellipse; if *ε *= 1, a parabola; and if *ε *> 1, a hyperbola.

Here we see the eccentricity vary from 0 to ∞. The focus and directrix are in red. The parabola is the “boundary” between the family of hyperbolas and the family of ellipses, and occurs when the eccentricity is 1.

You may have observed that the ellipse becomes more and more “circular” as we slide the eccentricity toward 0. When the eccentricity reaches 0, then the circle collapses to the point F itself. But if we “blow up” the curve as the eccentricity shrinks to 0, then the limiting curve is indeed a circle. So we may view the circle as part of the same family of curves.

Many of the important properties of circles generalize to more universal properties shared by the other conics. For instance, from any point in the exterior of a conic there are precisely two tangents; or again, at each point on a conic there is exactly one tangent. It is possible to prove these properties by analytical methods, but this lacks geometrical finesse, involving arguments that would have been alien to the Greeks. The most excellent way to prove many of the basic theorems about conics is to use the *eccentric circle*.

Consider the conic with focus *F*, directrix *l*, and eccentricity *ε*. The *interior* of the conic is defined as the set of points *P* for which *PF* is less than *ε* times the distance to *l*. Let *O* be any point not in the interior and not on the directrix, and let *s* be its perpendicular distance from *l*. Construct the circle centered at *O* with radius *ε*⋅*s*.

Now let *E* be any point on the circle. Let *R* be the point where the line *EF* intersects *l*. Let *P* be the point where the line through *F* parallel to the radius intersects *OR*. An argument from similarity shows that *P* is on the conic.

Now imagine letting *E* traverse the circle. Then the conic is swept out by the point *P* as shown.

Here we have an ellipse. If the conic is a parabola, then the directrix is tangent to the eccentric circle; if the conic is a hyperbola, then the directrix intersects the eccentric circles at two points, and the two branches of the hyperbola correspond to the two parts of the circle.

The eccentric circle is what allows us to prove theorems concerning tangent lines. We already know that from any point in the exterior of a circle there are two tangents to the circle. Given an eccentric circle centered at *O*, the two tangents from *F* to the circle produce two tangents to the conic as shown.

Similarly, to show that there is exactly one tangent at a point of the conic, construct the eccentric circle at *O*. This circle passes through *F*, and there is exactly one tangent to the circle at *F*; this tangent produces the tangent at *O*.

# Polygons and Divisibility

In the last post we introduced cyclic groups. The cyclic group C_{n} is generated by S, where S represents clockwise rotation through 360°/n (or 1/n of a revolution). It includes S^{2}, which is 2/n of a revolution, and S^{3}, which is 3/n of a revolution, and so on, all the way up to S^{n-1}. Notice that S^{n} = I is a complete revolution, which gets us back where we started.

More generally, suppose p is a whole number. Divide p by n to obtain the quotient q and remainder r. We can write p ÷ n = q R r or p = q ⋅ n + r. So S^{p} = S^{qn+r} = (S^{n})^{q} ∘ S^{r}. In other words, S^{p} consists of q complete revolutions composed with r/n of a revolution. It follows that S^{p} is the same as S^{r}. So, if we want to see what S^{p} does, we really only need to pay attention to the remainder of p ÷ n.

Suppose n = 8 for example. The group C_{8} is the group of rotations of the regular octagon, generated by a 45°-rotation S.

What transformation is (say) S^{29}? Well, 29 ÷ 8 = 3 R 5, so S^{29} amounts to 3 complete revolutions composed with 5/8 of a revolution, or rotation through 5 ⋅ 45° = 225°. In other words, S^{29} = S^{5}.

Now, we’ve noted that, if m is a divisor of n, then C_{m} is contained within C_{n}. This can be seen in the following way. Write n = m ⋅ q. Then S^{q} represents 1/m of a revolution. For instance, if n = 8, then we can take m = 4. Writing 8 = 4 ⋅ 2, we see that S^{2} represents one quarter of a revolution, i.e., a rotation through 90°.

This observation amounts to the following fact: a regular polygon with m sides can be inscribed in a regular polygon with n sides if and only if m is a divisor of n. (We regard a line segment as a “polygon” with two sides.) To draw the polygon with m sides, connect every q^{th} vertex, where n = m ⋅ q.

For instance, the divisors of 6 are 2 and 3, so we can inscribe an equilateral triangle and a bisecting line segment in a regular hexagon. The triangle is constructed by connecting every second vertex. Or again, the divisors of 12 are 2, 3, 4, and 6, so we can inscribe a hexagon, a square, an equilateral triangle, and a bisecting line segment in the regular dodecagon. The square is constructed by connecting every third vertex, since 12 = 4 ⋅ 3.

The divisors of 15 are 3 and 5, so we can inscribe a regular pentagon and an equilateral triangle in the regular pentadecagon. The pentagon is constructed by connecting every third vertex, since 15 = 5 ⋅ 3.

So, if m is a divisor of n with n = m ⋅ q, then 〈S^{q}〉 = C_{m}. We’d like to answer the more general question now: If p is any whole number, then what does the group 〈S^{p}〉 amount to? In other words, if we keep composing p/n of a revolution with itself, what cyclic group do we obtain?

Let’s consider the example of n = 8 again. Take p = 3, and write T = S^{3}. Then T is a 135°-rotation, or 3/8 of a revolution. Let’s start composing T with itself. To begin with, T^{2} is 6/8 of a revolution, or S^{6}. Then T^{3} is 9/8 of a revolution, which is the same as 1/8 of a revolution. So T^{3} = S. Next, T^{4} is 12/8 of a revolution, which is the same as 4/8 of a revolution, or S^{4}. Continuing like this, we find that the group generated by T consists of I, S^{3}, S^{6}, S, S^{4}, S^{7}, S^{2}, and S^{5}, in that order. So 〈T〉 = C_{8}.

Now let’s take p = 6. Write U = S^{6}. The group generated by U consists of S^{6}, S^{4}, S^{2}, and I. We don’t obtain the entire group in this case. In fact, since S^{6} is a 270°-rotation, S^{4} is a 180°-rotation, and S^{2} is a 90°-rotation, we see that 〈U〉 = C_{4}.

If we check each of the elements of C_{8}, what we’ll find is the following. The group can be generated by each of S, S^{3}, S^{5}, and S^{7}. The transformations S^{2} and S^{6} only generate C_{4}. The transformation S^{4} generates C_{2}. And the transformation I generates the trivial group C_{1}.

Now, if p and n are whole numbers not both zero, then their *greatest common divisor* d is the largest whole number that divides both p and n. For instance, the greatest common divisor of 6 and 8 is 2, while the greatest common divisor of 12 and 30 is 6. In general, the group generated by S^{p} is the same as the group generated by S^{d} where d is the greatest common divisor of n and p. So, writing n = m ⋅ d, we find that 〈S^{p}〉 = C_{m}. If p and n share no common divisors larger than 1, then they are said to be *relatively prime*; in this case, m = n, and S^{p} generates the whole cyclic group.

Consider the case n = 12. Then S, S^{5}, S^{7}, and S^{11} each generate C_{12} since 12 shares no common divisors larger than 1 with 1, 5, 7, or 11. The transformations S^{2} and S^{10} each generate C_{6} since the greatest common divisor of 12 and 2, or 12 and 10, is 2. The transformations S^{3} and S^{9} each generate C_{4} since the greatest common divisor of 12 and 3, or 12 and 9, is 3. The transformations S^{4} and S^{8} generate C_{3}. The transformation S^{6} generates C_{2}. And I generates C_{1}.

Or again, consider n = 15. Then S, S^{2}, S^{4}, S^{7}, S^{8}, S^{11}, S^{13}, and S^{14} each generate C_{15}. Next, S^{3}, S^{6}, S^{9}, and S^{12} each generate C_{5}. The transformations S^{5} and S^{10} generate C_{3}. And I generates C_{1}.

Pick a single vertex of the n-sided polygon. Imagine repeatedly applying a rotation S^{p} to it (where p is less than n) and connecting each pair of consecutive points with a line segment. This amounts to connecting every p^{th} vertex. If p happens to be 1 or n – 1, then we obtain the polygon itself. But if p is between 1 and n – 1, then we obtain either an inscribed polygon or a star. Also, the figure produced by p is the same as that produced by n – p; the direction is just reversed. It should also be clear that an inscribed star has n points if and only if p and n are relatively prime.

Everyone knows that there’s one 5-pointed star; this corresponds to p = 2 (or p = 3), because we draw it by connecting every second (or third) vertex.

Next, there are no 6-pointed stars because each of 2, 3, and 4 share common divisors with 6. But there are two 7-pointed stars, corresponding to p = 2 (or p = 5) and p = 3 (or p = 4).

We draw the first by connecting every second vertex, and the second by connecting every third vertex. There is only one 8-pointed star, corresponding to p = 3 (or p = 5).

It’s drawn by connecting every third vertex. If we connect every second vertex, we wind up with a square; if we connect every fourth vertex, we obtain a bisecting line segment. Next, there are two 9-pointed stars, corresponding to p = 2 (or p = 7) and p = 4 (or p = 5).

In general, if n happens to be a prime number, hence has no divisors other than 1 and itself, then there are (n – 3)/2 different n-pointed stars. For instance, there are (11 – 3)/2 = 4 different 11-pointed stars.

# Rotational Ornaments

An *isometry* is a transformation of the plane that takes each line segment to a congruent line segment and each angle to a congruent angle. Basic types of isometries include reflections, rotations, and translations.

In the last last post, we saw that a symmetry of a geometrical figure is an isometry that preserves the figure. A *group* is a collection of symmetries satisfying: (1) if a symmetry T is in the group, then its inverse is also in the group; and (2) if T and U are in the group, then the composition U ∘ T is also in the group.

We discussed the dihedral groups D_{3} of the equilateral triangle and D_{4} of the square. Now consider the set of symmetries that preserves the regular pentagon.

In this case we have five reflections: R_{1}, R_{2}, R_{3}, R_{4}, and R_{5}. In addition, we have clockwise rotation S through one fifth of a complete revolution, or 72°, and its compositions with itself: S^{2}, S^{3}, S^{4}. So the dihedral group D_{5} consists of ten symmetries: I, S, S^{2}, S^{3}, S^{4}, R_{1}, R_{2}, R_{3}, R_{4}, and R_{5}. Once again, we can write D_{5} = 〈R_{1},R_{2}〉, where R_{1} and R_{2} are reflections across adjacent lines making an angle of 36° with one another. For instance, R_{1} ∘ R_{2} is a 72°-rotation.

Now imagine extending the sides of the pentagon as shown.

This “breaks” the symmetry. No longer can we reflect. However, the rotations still preserve the figure. So the symmetry group now consists of five symmetries: I, S, S^{2}, S^{3}, and S^{4}. This is referred to as the *cyclic group* C_{5}. It’s generated by S, so we can write C_{5} = 〈S〉. The cyclic group is a subgroup of the dihedral group.

In general, the cyclic group C_{n} is generated by a rotation through 360°/n, or one n^{th} of a revolution, while the dihedral group D_{n} is generated by two reflections whose lines make an angle of 360°/2n. Cyclic and dihedral groups abound in ornamental symmetry; Owen Jones’ 1910 *Grammar of Ornament* contains a wealth of examples.

The group D_{1} contains a single reflection; it may be thought of as the set of symmetries of the letter **V**. This is bilateral symmetry, as exhibited in the following Egyptian ornament.

The group D_{2} contains two reflections across perpendicular lines; it may be thought of as the set of symmetries of the letter **H**. We see examples of it in the following Greek and Pompeian ornaments.

Notice that if we ignore the color in the Pompeian ornament, then we have a D_{4} symmetry instead. A true example of D_{4} symmetry is given in the following Persian ornament.

Larger dihedral groups are rarer but still occur. The following ornaments are, respectively, Assyrian, Chinese, and Byzantine; they have symmetry groups D_{5}, D_{8}, and D_{22}.

Ornaments that exhibit only a cyclic symmetry generally have some kind of overlapping knot pattern or swirl element which prevents reflection. The cyclic group C_{1} is the trivial group consisting of just the identity.

The cyclic group C_{2} contains a single rotation through 180°. This may be thought of as the set of symmetries of the letter **N**. The following Celtic ornament has C_{2} symmetry; the overlapping of the knot prevents its having D_{2} symmetry.

The cyclic group C_{4} is generated by a 90°-rotation. Examples include the following medieval European and Moorish ornaments.

The group C_{6} is generated by a 60°-rotation, while C_{8} is generated by a 45°-rotation; these are exhibited in the following medieval European and Arabian ornaments.

In the latter case, we have to ignore the box design around the circular ornament. If we don’t, then the symmetry group is C_{4} rather than C_{8}. In general, C_{n} contains C_{m} as a subgroup if and only if m is a divisor of n. So C_{8} contains C_{1}, C_{2}, and C_{4}. Similarly, C_{6} contains C_{1}, C_{2}, and C_{3}. We’ll return to this in the next post on the topic.

# Symmetry Groups

Think of a spacial arrangement of geometrical elements. It could be a polygon, or a honeycomb lattice, or some other arrangement. A *symmetry* is a rigid transformation that preserves the arrangement. Consider an equilateral triangle, for instance:

There are six “moves” that would preserve the triangle. To begin with, we could reflect it across any of the three axes of symmetry.

In addition, we could spin it through 120° around the center, clockwise or counterclockwise. And, finally, we could just leave it be.

Symmetries have the following property: if we perform two symmetries in succession, then the *composition* obtained is also a symmetry. For instance, if we reflect the triangle across two lines of symmetry, then this amounts to a 120° rotation. In general, if T and U are two symmetries, then we’ll use the notation U ∘ T to represent the symmetry obtained by performing first T and then U. We can think of this as a “multiplication” of symmetries. Be careful, though: the order matters. The symmetry U ∘ T is not necessarily the same as T ∘ U.

To make all this clearer, let’s consider another example. A square has four lines of symmetry:

Reflection across any of these lines preserves the shape. Label the four reflections as R_{1}, R_{2}, R_{3}, and R_{4}. We can also rotate the square clockwise around the center through 90°, or 180°, or 270°. If S is the symmetry that rotates the square through 90°, then S ∘ S = S^{2} is rotation through 180°, and S ∘ S ∘ S = S^{3} is rotation through 270°. And, finally, there’s the identity symmetry, which just leaves the square as it is. We’ll call that I. Notice that S^{4} = I, since S^{4} amounts to a 360° rotation.

So we have eight symmetries in all: I, S, S^{2}, S^{3}, R_{1}, R_{2}, R_{3}, and R_{4}. As mentioned above, we can view composition of symmetries as a kind of “multiplication” on this set. However, it isn’t commutative: the order matters. To see this, consider S and R_{1}. Let’s see what S ∘ R_{1} does to the square:

This amounts to the transformation R_{4}. Now let’s see what R_{1} ∘ S does:

This amounts to R_{2}. So S ∘ R_{1} = R_{4} while R_{1} ∘ S = R_{2}. It follows that S ∘ R_{1} ≠ R_{1} ∘ S.

What *is* true is that the composition of any two symmetries in our set is also in the set. The set of symmetries is “closed” under composition. In addition, each symmetry has its inverse, which carries the shape back to the original position. For instance, R_{1} is its own inverse, since R_{1} ∘ R_{1} = I. The same is true of all the reflections. On the other hand, the inverse of S is S^{3}, since S ∘ S^{3} is a 360° rotation, which amounts to I. And, finally, the inverse of S^{2} is itself, since two 180° rotations amount to a 360° rotation. If we wanted to, we could make an 8 × 8 multiplication table for this set of symmetries. Each row and each column would contain each symmetry exactly once.

Another interesting thing to note is that we could use a pair of reflections to obtain the other symmetries. Take R_{1} and R_{2}, for instance. Notice that S = R_{1} ∘ R_{2}. We can compose S with itself to obtain the other rotations: S^{2} = R_{1} ∘ R_{2} ∘ R_{1} ∘ R_{2} and S^{3} = R_{1} ∘ R_{2} ∘ R_{1} ∘ R_{2} ∘ R_{1} ∘ R_{2}. Finally, R_{3} = R_{2} ∘ R_{1} ∘ R_{2}, while R_{4} = R_{1} ∘ R_{2} ∘ R_{1}. So we’ve written every symmetry as a product of R_{1} and R_{2}.

A *group* is a set of symmetry operations satisfying the following two requirements: (1) the composition of two operations in the group is also in the group, and (2) for each operation in the group, the inverse of the operation is also in the group. The group we’ve been discussing is the *dihedral group* D_{4}. It has order 8, meaning that it contains 8 operations: I, S, S^{2}, S^{3}, R_{1}, R_{2}, R_{3}, and R_{4}. We say that D_{4} is *generated by* R_{1} and R_{2}, and write D_{4} = 〈R_{1},R_{2}〉.

In coming posts, we’ll see how the theory of groups can be used to classify ornamental symmetries.

# Unity and Infinity

In recent weeks, Yahoo! News, that worthy and reliable news source, ran an article purporting to list the eleven most beautiful mathematical equations (ten having been too few). Alas, of the list, five are not mathematical, and one is not an equation. However, among the remaining five is the following:

where the 9’s are understood to keep going forever. In my experience, this equation prompts varied responses. The most common is sheer disbelief. “How could these two be equal?” people ask. The answer to this is that, if they aren’t equal, then we should be able to find their difference:

But these doubters of the mysteries of unity and infinity can’t state the answer. Generally they’ll admit that it must be a number smaller than every other number, but they still insist that it can’t be zero. What’s interesting is that they’re generally comfortable with the equation

After all, a calculator tells us it’s so. But if we multiply both sides by 3, then don’t we get the original?

It isn’t surprising that our equation seems a little dubious. What this really stems from is not understanding what infinite decimal expansions like .333… and .999… actually mean. Now, *terminating* decimal expansions are easy to understand. For instance,

But by what right can we write down something like

where the sum just keeps going and going? How can we add *infinitely* many numbers? Does it even make sense to talk about something like that?

To answer this, let’s consider the sequence of *partial* sums:

and so on. Notice that each term in the sequence is a bit larger than the preceding, and that all are less than 1:

Modern mathematics defines the meaning of the infinite decimal expansion as follows: the value of the expansion is the number x such that, for *any* positive number ϵ, no matter how small, our sequence of partial sums is eventually in between x – ϵ and x. Put in another way, the value is x if, for any small number ϵ, the difference between x and the n^{th} partial sum is smaller than ϵ if n is large enough.

So, you see, the admission that the difference between 1 and .999… is a number “smaller than every other number” means that .999… is in fact *equal* to 1.