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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 begin{equation} label{eq:poly} 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: begin{equation} label{eq:sys} left{ P_{N-1}(x_k) = f_kright},quad k=-frac{N-1}{2},dots,frac{N-1}{2} end{equation} Here are references to existing equations: (ref{eq:poly}), (ref{eq:sys}). Here is reference to non-existing equation (ref{eq:unknown}).