Calculate IFFT using only FFT?
We want to do inverse DFT:
$$
x(k) = \frac{1}{\sqrt{N}}\sum_n y(n) e^{j 2\pi kn/N},
$$
but can only do forward DFT. So let's try to make it "look" like a forward DFT. Note that we want a minus sign in the complex exponential. We can introduce it via complex conjugation:
$$
\overline{x(k)} = \frac{1}{\sqrt{N}}\sum_n \overline{y(n)} e^{-j 2\pi kn/N},
$$
That is starting to look like a forward DFT now! So the recipe is:


Complex conjugate the given sequence that we want to inverse DFT
Calculate its forward DFT
Calculate complex conjugate of the result. 


That gives you the inverse DFT of the original sequence.

(Note that I cheated a little bit on the scaling factor to make it easier!)