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$$ f(t_l)={1 \over N}\sum_{q=0}^{q=N-1}f_q*e^{(2*\pi*i*q*l/N)},$$
\\ $$f_q={1 \over \Delta
t}(c_q+c_{q+N}+c_{q-N}+c_{q+2*N}+c_{q-2*N}+...)
\hspace{1cm}(3) $$
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