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We give a time regularity result for an abstract mixed type equation whose toy model may be ${(ru)}_t-\mathrm{\Delta }u=f$ where $r$ is a coefficient whose sign may be positive, null and negative.

We study the asymptotic behaviour of a sequence of strongly degenerate parabolic equations ${\partial }_t\left(r_{h}u\right)-\mathrm{div}(a_h·Du)$ with $r_h(x,t)\ge 0$, $r_h\in L^{\infty }(\Omega \times (0,T))$. The main problem is the lack of compactness, by-passed via a regularity result. As particular cases, we obtain -convergence for elliptic operators $(r_h\equiv 0)$, -convergence for parabolic operators $(r_h\equiv 1)$, singular perturbations of an elliptic operator $(a_h\equiv a$ and $r_h\to r$, possibly $r\equiv 0)$.

We prove a characterisation of sets with finite perimeter and $BV$ functions in terms of the short time behaviour of the heat semigroup in ${\mathbf{R}}^N$. For sets with smooth boundary a more precise result is shown.