I'm having some trouble with this derivation
$$s_d = \begin{cases}1&\text{for}\quad m\quad\text{multiples of}\quad D\\ 0 &\text{otherwise} \end{cases}$$
then $s_d$ is somehow rewritten to:
$$\frac{1}{D}\sum_{k=0}^{D-1}e^{jk2\pi \frac{m}{D}} $$
Can someone explain this?
EDIT:
Basically the same derivation as done here Frequency Representation of Downsampled Signal