$$ \newcommand{\uex}{{u_{\small\mbox{e}}}} \newcommand{\Aex}{{A_{\small\mbox{e}}}} \newcommand{\half}{\frac{1}{2}} \newcommand{\tp}{\thinspace .} \newcommand{\Oof}[1]{\mathcal{O}(#1)} \newcommand{\dfc}{\alpha} % diffusion coefficient $$

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Fourier representation

Represent \( I(x) \) as a Fourier series $$ I(x) \approx \sum_{k\in K} b_k e^{ikx} $$

The corresponding sum for \( u \) is $$ u(x,t) \approx \sum_{k\in K} b_k e^{-\dfc k^2t}e^{ikx} $$

Such solutions are also accepted by the numerical schemes, but with an amplification factor \( A \) different from \( \exp{({-\dfc k^2t})} \): $$ u^n_q = A^n e^{ikq\Delta x} = A^ne^{ikx} $$

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