In mathematics, a complex number is an expression of the form a + bi, where a and b are real numbers and i represents the imaginary number defined as i^2 = -1. (In other words, i is the square root of -1.) The real number a is called the real part of the complex number, and the real number b is the imaginary part. For example, 3 + 2i is a complex number, having real part 3 and imaginary part 2. Complex numbers are often used in applied mathematics, control theory, signal analysis, fluid dynamics and other fields.