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26 At Right Angles | Vol. 3, No. 1, March 2014
Invariance can also be seen in reflection. Figure
3 gives a design for the word “algebra” that is
invariant upon reflection, but with a twist. You
will notice that the left hand side is NOT the same
as the right hand side and yet the word is still
readable when reflected. So the invariance occurs
at the level of meaning even though the design is
not visually symmetric!
Figure 3. An ambigram for ‘algebra’ that remains
invariant on reflection. But is it really symmetric?
In this column, we use ambigrams to demonstrate
(and play with) mathematical ideas relating to
symmetry and invariance.
There are two common ways one encounters
symmetry in mathematics. The first is related to
graphs of equations in the coordinate plane, while
the other is related to symmetries of geometrical
objects, arising out of the Euclidean idea of
congruence. Let’s take each in turn.
Symmetries of a Graph
First let us examine the notions of symmetry
related to graphs of equations and functions.
All equations in x and y represent a relationship
between the two variables, which can be plotted
on a plane. A graph of an equation is a set of points
(x, y) which satisfy the equation. For example,
x2 + y2 = 1 represents the set of points at a distance
1 from the origin—i.e. it represents a circle.
Let (a, b) be a point in the first quadrant. Notice
that the point (–a, b) is the reflection of (a, b) in
the y-axis (See Figure 4a). Thus if a curve has the
property that (–x, y) lies on the curve whenever
(x, y) does, it is symmetric across the y-axis. Such
functions are known as even functions, probably
because y = x2
, y = x4
, y = x6
,... all have this
property. Figure 4b shows the graph of y = x2
is an even function whose graph is a parabola.
Similarly, a curve is symmetrical across the origin
if it has the property that (–x, –y) lies on the curve
whenever (x, y) does. Functions whose graph
is of this kind are called odd functions, perhaps
because y = x, y = x3
, y = x5
,... all have this property.
See Figure 4c for an odd function.
A graph can also be symmetric across the x-axis.
Here (x, –y) lies on the curve whenever (x, y)
does. The graph of the equation x = y2
parabola) is an example of such a graph. Can a
(real) function be symmetric across the x-axis?
Figure 5 shows a chain ambigram for “parabola”.
Compare the shape of this ambigram with the
graph in Figure 4b. The chain extends indefinitely,
just like the graph of the underlying equation!
Figure 5. A parabolic chain ambigram for “parabola”
Figure 4a. The point A (a, b)
and some symmetric points
Figure 4b. An even function:
y = x2
Figure 4c. An odd function: y = x3
Figure 2. An ambigram for “invariant”
that remains invariant on rotation
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At Right Angles | Vol. 3, No. 1, March 2014 27
The ambigram for “axis of symmetry” (Figure 6)
is symmetric across the y-axis. You can see a red y
as a part of the x in the middle. So this ambigram
displays symmetry across the y axis. At the same
time it is symmetric across the letter x!
Figure 6. The axis of symmetry: Is it the y-axis or the x?
Another possibility is to interchange the x and y
in an equation. Suppose the original curve is C1
and the one with x and y interchanged is C2. Thus
if (x, y) is a point on C1, then (y, x) lies on C2. By
looking at Figure 4a, convince yourself that the
point (b, a) is the reflection of (a, b) in the line y
= x, the straight line passing through the origin,
and inclined at an angle of 45°
to the positive side
of the x-axis. Thus the curve C2 is obtained from
C1 by reflection across the line y = x. If C1 and C2
(as above) are both graphs of functions, then
they are called inverse functions. An example of
such a pair: the exp (exponential y = ex
) and log
(logarithmic y = ln x) functions (Figure 7).
Figure 7. Inverse functions are symmetric across the
Figure 8 is a remarkable design that where exp
becomes log when reflected in the line y = x.
Figure 8: Exp becomes Log when reflected
across the diagonal line!
A great example of an inverse function is the
hyperbola y = 1 / x, defined for all non-zero real
numbers x (Figure 9). Its inverse is obtained
by interchanging x with y. But x = 1 / y can be
written y = 1 / x. So it is its own inverse, and thus
symmetric across the line y = x.
Figure 9. The symmetrical graph of the hyperbola. It is
its own inverse. And it’s odd, too!
of symmetry of symmetry
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28 At Right Angles | Vol. 3, No. 1, March 2014
The ambigram for “inverse” in Figure 10 is
inspired by the hyperbola.
Figure 10. An ambigram for the word “inverse”
shaped like a hyperbola.
It is symmetric across the origin and across the
line joining the two S's.
Another type of symmetry consideration arises
from the notion of congruence in plane geometry.
Two objects are considered to be congruent if one
object can be superposed on the other through
rotation, reflection and/or translation.
Figure 11 shows an ambigram of the word
Figure 11. An ambigram for “rotate”. What happens
when you rotate it through 180°
This leads to the question: if we can rotate “rotate”
can we reflect “reflect”? Figure 12 is an ambigram
for “reflect” that is symmetric around the vertical
line in the middle.
Figure 12. An ambigram for “reflect.” What happens
when you hold it to a mirror?
Finally, the third operation is translation. An
example of this symmetry is shown by the chain
ambigram for sine in Figure 13.
Figure 13. A sine wave ambigram. It displays
The sine function satisfies many symmetry
properties. Perhaps the most important of them
is that it is periodic, i.e., if you shift (in other
words, translate) the functions by 2π, then you get
the same function back. In addition, it is an odd
function, and the ambigram is both periodic and
One can of course combine these transformations.
This is best understood by looking at the
symmetries of an equilateral triangle (see Figure
14) involving both rotation and reflection.
Figure 14. The 6 symmetries of an equilateral triangle.