Motivated by applications to gas filtration problems, we
study the regularity of weak solutions to the strongly degenerate
parabolic PDE
u
t
-
div
(
(
|
D
u
|
-
ν
)
+
p
-
1
D
u
|
D
u
|
)
=
f
in
Ω
T
=
Ω
×
(
0
,
T
)
,
u_{t}-\operatorname{div}\Bigl{(}(\lvert Du\rvert-\nu)_{+}^{p-1}\frac{Du}{%
\lvert Du\rvert}\Bigr{)}=f\quad\text{in }\Omega_{T}=\Omega\times(0,T),
where Ω is a bounded domain in
ℝ
n
{\mathbb{R}^{n}}
for
n
≥
2
{n\geq 2}
,
p
≥
2
{p\geq 2}
, ν is a positive constant and
(
⋅
)
+
{(\,\cdot\,)_{+}}
stands for the positive part. Assuming that the datum f belongs
to a suitable Lebesgue–Sobolev parabolic space, we establish the Sobolev
spatial regularity of a nonlinear function of the spatial gradient
of the weak solutions, which in turn implies the existence of the
weak time derivative
u
t
{u_{t}}
. The main novelty here is that the structure
function of the above equation satisfies standard growth and ellipticity
conditions only outside a ball with radius ν centered at the
origin. We would like to point out that the first result obtained
here can be considered, on the one hand, as the parabolic counterpart
of an elliptic result established in [L. Brasco, G. Carlier and F. Santambrogio,
Congested traffic dynamics, weak flows and very degenerate elliptic equations [corrected version of mr2584740],
J. Math. Pures Appl. (9) 93 2010, 6, 652–671], and on the other
hand as the extension to a strongly degenerate context of some known
results for less degenerate parabolic equations.