A criterion for the existence of T-periodic solutions of nonautonomous parabolic equation $$u_t = \Delta u + V(x)u + f(t,x,u)$$
u
t
=
Δ
u
+
V
(
x
)
u
+
f
(
t
,
x
,
u
)
, $$x\in {\mathbb {R}}^N$$
x
∈
R
N
, $$t>0$$
t
>
0
, where V is Kato–Rellich type potential and f diminishes at infinity, will be provided. It is proved that, under the nonresonance assumption, i.e. $${\mathrm {Ker}} (\Delta + V)=\{0\}$$
Ker
(
Δ
+
V
)
=
{
0
}
, the equation admits a T-periodic solution. Moreover, in case there is a trivial branch of solutions, i.e. $$f(t,x,0)=0$$
f
(
t
,
x
,
0
)
=
0
, there exists a nontrivial solution provided the total multiplicities of positive eigenvalues of $$\Delta +V$$
Δ
+
V
and $$\Delta + V + f_0$$
Δ
+
V
+
f
0
, where $$f_0$$
f
0
is the partial derivative $$f_u(\cdot ,\cdot ,0)$$
f
u
(
·
,
·
,
0
)
of f, are different mod 2.