O 1/M é um fator de escala que pode ser opcionalmente utilizado na transformada. Bom, pelo menos foi o que eu entendi no Gonzalez:
The 1/M multiplier in front of the Fourier transform sometimes is placed in front of the inverse instead. Other times (not as often) both equations are multiplied by 1/sqrt(M). The locations of the multiplier does no matter. If two multipliers are used, the only requirement is that their produt be equal to 1/M,