i don't understand why it says " an infinite number of periods as t ranges from -oo to +oo " ?

isn't the period all be positive???

thanks

Periodic signals-and in particular, the complex periodic exponential signal in eq .21 and the sinusoidal signal in eq. (1.25-provide important examples of signals with infinite total energy but finite average power. For example, consider the periodic ex- ponential signal of eq. (1.21), and suppose that we calculate the total energy and average power in this signal over one period: To period = To Periodic signals-and in particular, the complex periodic exponential signal in eq .21 and the sinusoidal signal in eq. (1.25-provide important examples of signals with infinite total energy but finite average power. For example, consider the periodic ex- ponential signal of eq. (1.21), and suppose that we calculate the total energy and average power in this signal over one period: To period = To