C[1,3] refers to a specific subset of the set of complex numbers, where the real part is 1 and the imaginary part is 3. In mathematical notation, this can be expressed as the complex number 1 + 3i, where "i" represents the imaginary unit, defined as the square root of -1. This notation is often used in fields such as mathematics, engineering, and physics to represent points in the complex plane. The complex plane is a two-dimensional space where the x-axis represents the real part and the y-axis represents the imaginary part of complex numbers.