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© 1999-2010 by Francis Edward Su

From the Fun Fact files, here is a Fun Fact at the Medium level:

# i to the i is a Real Number

If you are familiar with complex numbers, the "imaginary" number i has the property that the square of i is -1. It is a rather curious fact that i raised to the i-th power is actually a real number!

In fact, its value is approximately 0.20788.

Presentation Suggestions:
This makes a great exercise after learning the basics about complex numbers.

The Math Behind the Fact:
From Euler's formula, we know that exp(i*x) = cos(x) + i*sin(x), where "exp(z)" is the exponential function ez. Then

exp(i*Pi/2) = cos(Pi/2) + i*sin(Pi/2) = i.

Raising both sides to i-th power, we see that the right side is the desired quantity ii, while the left side becomes exp(i*i*Pi/2), or exp(-Pi/2), which is approximately .20788.

(Actually, this is one of many possible values for i to the i, because, for instance, exp(5i*Pi/2)=i. In complex analysis, one learns that exponentiation with respect to i is a multi-valued function.)

Su, Francis E., et al. "i to the i is a Real Number." Math Fun Facts. <http://www.math.hmc.edu/funfacts>.

References:

Keywords:    i^i, i to the i-th power, i raised to the i, imaginary power
Subjects:    calculus, analysis
Level:    Medium
Fun Fact suggested by:   Ed Poncin
Suggestions? Use this form.
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