The function exp(-ax), where a is a positive constant, is considered piecewise smooth because it is smooth everywhere, with no points of discontinuity. This aligns with the definition of piecewise smooth, which requires the function to be smooth except at finitely many points. Since exp(-ax) has zero points of non-smoothness, it meets this criterion. Therefore, it can be used to find a Fourier series representation. The discussion confirms that exp(-ax) is indeed piecewise smooth.