$\vartheta_t = (D(\vartheta)\vartheta_x)_x + f(\vartheta)$
with respect to both the reaction term $f$ and the
diffusivity $D$.
We also introduce and discuss the concept of fast
heteroclinic in this context, which allows to interpret the
appearance of sharp heteroclinic in the case of degenerate
diffusivity ($D(0)=0)$.
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