# 14.2.5: Complex Numbers

• Franz S. Hover & Michael S. Triantafyllou
• Massachusetts Institute of Technology via MIT OpenCourseWare

#### Complex Numbers

The complex number $$z = x + iy$$ is interpreted as follows: the real part is $$x$$, the imaginary part is $$y$$, and $$i = \sqrt{−1}$$ (imaginary). DeMoivre’s theorem connects complex $$z$$ with the complex exponential. It states that $$cos \, \theta + i \, sin \, \theta = e^{i \theta}$$, and so we can visualize any complex number in the two-dimensional plane, where the axes are the real part and the imaginary part. We say that $$Re(e^{i \theta}) = cos \, \theta$$, and $$Im(e^{i \theta}) = sin \, \theta$$, to denote the real and imaginary parts of a complex exponential1. More generally, $$Re(z) = x$$ and $$Im(z) = y$$.

A complex number has a magnitude and an angle: $$|z| = \sqrt{x^2 + y^2}$$, and2 arg $$(z) = arctan2(y, x)$$. We can refer to the $$[x, y]$$ description of $$z$$ as Cartesian coordinates, whereas the [magnitude, angle] description is called polar coordinates. This latter is usually written as $$z = |z| \angle$$ arg$$(z)$$. Arithmetic rules for two complex numbers $$z_1$$ and $$z_2$$ are as follows:

\begin{align*} z_1 + z_2 &= (x_1 + x_2) + i (y_1 + y_2) \\ z_1 - z_2 &= (x_1 - x_2) + i (y_1 - y_2) \\ z_1 * z_2 &= |z_1| |z_2| \angle \arg (z_1) + \arg (z_2) \\ z_1 / z_2 &= \dfrac{|z_1|} {|z_2|} \angle \arg (z_1) - \arg (z_2) \end{align*}

Note that, as given, addition and subtraction are most naturally expressed in Cartesian coordinates, and multiplication and division are cleaner in polar coordinates.

• Franz S. Hover & Michael S. Triantafyllou
• Some modification and additions of this section were done by Scott D. Johnson, Joshua Halpern, and Scott Sinex. The majority of the work is however Hover's and Triantafyllou's

1Note also that we can now define sine and cosine as $$sin(\theta) = \frac{e^{i\theta} - e^{-i\theta}}{2i}$$ and $$cos(\theta) = \frac{e^{i\theta} + e^{-i\theta}}{2}$$. Later you can compare this to the hyperbolic sine and cosine in terms of exponents.

2Arctan2 is computerese for a two argument arctangent to ensures a unique arctangent for the ratio $$\frac{y}{x}$$. This was introduced into FORTRAN early on in its development. Other programming languages adopted this notation and eventual different fields not related to programming adopted the terminology as well. Run the program below to help see what this is all about.

y = 5;
x = 6;
printf("atan and atan2 with values y = %f and x = %f\n",y,x)
printf("atan(y/x) = %f\n",atan(y/x))
printf("atan2(y,x) = %f\n",atan2(y,x))
y = -5;
x = -6;
printf("atan and atan2 with values y = %f and x = %f\n",y,x)
printf("atan(y/x) = %f\n",atan(y/x))
printf("atan2(y,x) = %f\n",atan2(y,x))

14.2.5: Complex Numbers is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by Franz S. Hover & Michael S. Triantafyllou (MIT OpenCourseWare) via source content that was edited to conform to the style and standards of the LibreTexts platform; a detailed edit history is available upon request.