9. Even and Odd Functions
By M. Bourne
Even Functions
A function y = f(t) is said to be even if
f(-t) = f(t)
for all values of t.
The graph of an even function is always symmetrical about the vertical axis (that is, we have a mirror image through the y-axis).
The waveforms shown below represent even functions:
Cosine curve
f(t) = 2 cos πt
Notice that we have a mirror image through the f(t) axis.
Even Square wave:
Triangular wave:
In each case, we have a mirror image through the f(t) axis. Another way of saying this is that we have symmetry about the vertical axis.
Odd Functions
A function y = f(t) is said to be odd if
f(-t) = - f(t)
for all values of t.
The graph of an odd function is always symmetrical about the origin.
Origin Symmetry
A graph has origin symmetry if we can fold it along the vertical axis, then along the horizontal axis, and it lays the graph onto itself.
Another way of thinking about this is that the graph does exaclty the opposite thing on each side of the origin. If the graph is going up to the right on one side of the origin, then it will be going down to the left by the same amount on the other side of the origin.
Examples of Odd Functions
The waveforms shown below represent odd functions.
Sine Curve
y(x) = sin x
Notice that if we fold the curve along the y-axis, then along the t-axis, the graph maps onto itself. It has origin symmetry.
"Saw tooth" wave
Odd Square wave
Each of these three curves is an odd function, and the graph demonstrates symmetry about the origin.
Exercise
Sketch each function and then determine whether each function is odd or even:
(a) 
(b)
and f(t) = f(t + 2π)
(This last line means: Periodic with period = 2π)
(c)
(d)
(e)
(f)
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