The Dirac delta function δ( t ) and the Heaviside unit step function θ( t ) are known as singularity functions. By definition, a stable function has finite energy. A stable time function h ( t ) and a stable space function g ( x ) have well-defined Fourier transforms, designated by H (ω) and G ( k ), respectively. However, the study of wave propagation involves functions that are not stable. For example, the function sin k 0 x does not have finite energy, so it is not stable. The Fourier transforms of such unstable functions are not well defined. Thus, in wave propagation, we must consider using quantities that involve a certain kind of infinity.