# Eye to Eye: Imaginary Exponentiation

The term “imaginary number” is an unfortunate one. It makes these numbers seem strange and separate from more familiar “real” numbers, when in fact there is very little difference. I prefer the term complex numbers that encompasses the closed set of all real and imaginary numbers with the usual arithmetic operators. Recall that the imaginary numbers are numbers that are less then zero when squared, with the imaginary constant i representing the square root of -1:

i 2 = -1

One can add, subtract, multiply and divide with it just like other numbers. One can not only square it to get -1, but also take its square root, which turns out to be another complex number.

i  = 2/2 + i 2/2

But what about raising i to the ith power?

Surely, that must be some sort of weird “very imaginary” number, right? But in fact, it is just a real number, approximately 0.2078796…

The same mechanism that allows us to take the square root of i can be used to explain why ii is real. Just as real numbers can be visualized on the familiar number line, complex numbers can be represented by a plane where the horizontal axis represents real numbers and the vertical axis represents imaginary numbers.

Any complex number x + yi can also be expressed with an angle and a radius: rcosθ+risinθ. Using the angular representation on the plane, we can then visualize any exponentiation operation (take the square, the square root, etc.) as a rotation around the origin.

Squaring a number means doubling the angle. Taking the square root means cutting the angle in half. The imaginary constant i has a radius of 1 and an angle of 90 degrees (or π/2 radians). Doubling it to 180 degrees rotates to the position of -1 on the complex plane. SImilarly, taking the square root of i reduces the angle to 45 degrees, moving it into the position of 2/2 + i 2/2.

But how does one rotate an angle by an imaginary amount? To accomplish this, we turn to one of my favorite formulas in all of mathematics, Euler’s identity:

e = cosθ+isinθ

This identity unites trigonometry and exponentiation using the complex plane and rotations. It is more than just a curiosity and has practical applications including signal processing that we use for synthesizers and audio effects. However, it does allow us to also calculate the value of ii:

ii = cos(πi/2) + isin(πi/2) = eiπi/2 = e-π/20.20787957635076193…

It is odd how rotating an imaginary number by an imaginary factor yields a real number.

# Pi Day 3.14159…

We at CatSynth once again, celebrate Pi Day on its three-digit approximation, March 14 (3-14).

We start with some interesting facts about the digits of pi. We presented statistics about the distribution in our 2007 Pi Day post. From super-computing.org, we present some interesting patterns:

01234567890 first occurs at the 53,217,681,704-th digit of pi.
09876543210 first occurs at the 42,321,758,803-th digit of pi.
777777777777 first occurs at the 368,299,898,266-th digit of pi.
666666666666 first occurs at the 1,221,587,715,177-th digit of pi.
271828182845 first occurs at the 1,016,065,419,627-th of digit pi. (that’s e for those who haven’t memorized it)
314159265358 first occurs at the 1,142,905,318,634-th digit of pi.

Last year, we showed the relationship to the Gamma function, and of course to Euler’s identity, which links pi surprisingly closely to the imaginary constant i and the number e. But it is also surprisingly easy to generate pi from simple sequences of integers. Consider the Madhava-Leibniz formula for pi:

Thus one can generate pi from odd integers and simple arithmetic. Another formula only involving perfect squares of integers comes from the Basel problem (named for the town of Basel in Switzerland):

In recognition of Pi Day, the U.S. House of Representatives passed a resolution this week:

And thus the sad history of pi in politics as exemplified by the Indiana Pi Bill of 1897 is put to rest. Now onto erasing the sad history of science and politics in general of the past eight years…

# Fun with Pi (Day)

We saw this picture on meeyauw, and thought it was a good way to open our own Pi Day offering. Pi Day, or π day is celebrated on March 14 (3/14) of every year.

π does turn up in some interesting places besides circles and standard trigonometry (and LOLcat photos). There is of course Euler's famous identity:

which unites π with four of the other most famous constants in mathematics: zero, 1, i (the imaginary root of -1) and e. But it also turns up in some more surprising places. Consider the well-known factorial function, where n! or “n factorial” is the product of all the integers between 1 and n. For example:

### 5! = 5×4×3×2×1 = 120.

Simple enough. But of course some troublemaker is eventually going to ask for the factorial of 1/2. Not so easy. Fortunately, there is a function, called the Gamma function, that provides a solution:

Not really as simple as the original integer-only factorial. Once calculus is involved, might as well forget about it. But if you go through the trouble of plugging in 1/2 to the formula, you get the following intriguing result:

or

So the factorial of one half is one half the square root of π. Who knew?