Classical simulability and the significance of modular exponentiation in Shor’s algorithm

We show that a classical algorithm efficiently simulating the modular exponentiation circuit, for certain product state input and for general product measurements at the output, can efficiently simulate Shor's factoring algorithm. This is done by using the notion of the semiclassical Fourier transform due to Griffith and Niu, and further discussed in the context of Shor's algorithm by Browne.