Previously, we focussed on evolutionary equations of the form
$$\displaystyle \left (\overline {\partial _{t,\nu }M(\partial _{t,\nu })+A}\right )U=F. $$
∂
t
,
ν
M
(
∂
t
,
ν
)
+
A
¯
U
=
F
.
In this chapter, where we turn back to well-posedness issues, we replace the material law operator M(∂t,ν), which is invariant under translations in time, by an operator of the form
$$\displaystyle \mathcal {M}+\partial _{t,\nu }^{-1}\mathcal {N}, $$
ℳ
+
∂
t
,
ν
−
1
N
,
where both
$$\mathcal {M}$$
ℳ
and
$$\mathcal {N}$$
N
are bounded linear operators in
$$L_{2,\nu }(\mathbb {R};H)$$
L
2
,
ν
(
ℝ
;
H
)
. Thus, it is the aim in the following to provide criteria on
$$\mathcal {M}$$
ℳ
and
$$\mathcal {N}$$
N
under which the operator
$$\displaystyle \partial _{t,\nu }\mathcal {M}+\mathcal {N}+A $$
∂
t
,
ν
ℳ
+
N
+
A
is closable with continuous invertible closure in
$$L_{2,\nu }(\mathbb {R};H)$$
L
2
,
ν
(
ℝ
;
H
)
. In passing, we shall also replace the skew-selfadjointness of A by a suitable real part condition. Under additional conditions on
$$\mathcal {M}$$
ℳ
and
$$\mathcal {N}$$
N
, we will also see that the solution operator is causal. Finally, we will put the autonomous version of Picard’s theorem into perspective of the non-autonomous variant developed here.