Equation 4.3a uses unit vectors to represent the x and y vector component directions in the cartesian from. Figure 4-7 shown the unit vector notation for a position vector. Equation 4.3b uses complex number notation where in the X direction is called the real portion and the Y direction component is called the imaginary portion. This unfortunate term imaginary comes about because of the notation j to represent the square root of minus one, which of course cannot be evaluated numerically.