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For admissible initial data, for example a square-integrable function on the interval, expand in the sine eigenfunctions of the Dirichlet Laplacian:The expansion follows either from separation of variables or the complete sine orthogonal basis. If $\kappa <{\pi }^2$, every growth rate is negative and Parseval's identity givesThus all finite-energy data decay. The convergence is also uniform in $x$ after any fixed positive time: apply Cauchy-Schwarz inequality to the sine series, using the summable factors $e^{-2(j^2-1){\pi }^{2}t}$, to bound its supremum by a constant times $e^{-({\pi }^2-\kappa )t}$ for $t\ge t_0>0$.

Conversely choose $u(x,0)=\sin \pi x$. Then $u(x,t)=e^{(\kappa -{\pi }^2)t}\sin \pi x$, which is stationary at equality and grows when $\kappa >{\pi }^2$. All initial conditions decay if and only if $\kappa <{\pi }^2$. This is the continuous part of the reaction-diffusion spectral decay threshold.

Write $h=\Delta x=1/(M+1)$. The semidiscrete matrix is $B=h^{-2}\mathrm{tridiag}(1,-2,1)+\kappa I$. For $j=1,\dots ,M$, set $v_m^{(j)}=\sin (j\pi m/(M+1))$. The endpoint values vanish and the sine addition formula givesThese form an orthogonal basis of eigenvectors. Every initial vector decays precisely when every eigenvalue is negative. Since the largest is ${\lambda }_1$, the necessary and sufficient condition isNecessity follows by taking the first eigenvector as the initial data; at equality it remains unchanged. Sufficiency follows by decomposing any vector into the displayed decaying modes. The discrete reaction-diffusion spectral decay threshold is below ${\pi }^2$, since $\sin x<x$ for $x>0$, and equals ${\pi }^2-{\pi }^4{h}^2/12+O(h^4)$ as the mesh is refined. Therefore a coarse semidiscretization can spuriously grow for some parameters for which the continuous equation still decays.