(where the squared-norm notation will be clearer after the next Chapter). This definition is consistent with the idea that, if the values of the sequence represent a time-varying voltage, the above sum would express the total energy (in joules) dissipated over a 1Ω-resistor. Obviously, the energy is finite only if the above sum converges, i.e. if the sequence x[n] is square-summable. A signal with this property is sometimes referred to as a finite- energy signal. For a simple example of the converse, note that a periodic signal which is not identically zero is not square-summable.