The discrete Fourier coefficients are given by the formula:
$$c_k = \frac{1}{n}\sum_{j=0}^{n-1} f(x_j)e^{-2\pi ijk/n}$$
For $n=4$, we have the interpolation points $x_0=0$, $x_1=1$, $x_2=2$, and $x_3=3$. Plugging these values into the formula, we get:
$$c_0 =
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