Math Test - Crownstar

At first, we sample $f(x)$ in the $N$ ( $N$ is odd) equidistant points around $x^$ :

$f_k = f(x_k),\: x_k = x^+kh,\: k=-\frac{N-1}{2},\dots,\frac{N-1}{2}$

where $h$ is some step.
Then we interpolate points ${(x_k,f_k)}$ by polynomial

(1) $\begin{equation*} P_{N-1}(x)=\sum_{j=0}^{N-1}{a_jx^j}\end{equation*}$

Its coefficients ${a_j}$ are found as a solution of system of linear equations:

(2) $\begin{equation*} \left{ P_{N-1}(x_k) = f_k\right},\quad k=-\frac{N-1}{2},\dots,\frac{N-1}{2}\end{equation*}$

Here are references to existing equations: (1), (2).
Here is reference to non-existing equation (??).

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