We investigate some spectral properties of differential–difference operators, which are symmetrizations of differential operators of the form $$(\mathfrak {d}^\dagger \mathfrak {d})^k$$
(
d
†
d
)
k
and $$(\mathfrak {d}\mathfrak {d}^\dagger )^k$$
(
d
d
†
)
k
, $$k\geqslant 1$$
k
⩾
1
. Here, $$\mathfrak {d}=p\frac{d}{\textrm{d}x} +q$$
d
=
p
d
d
x
+
q
and $$\mathfrak {d}^\dagger $$
d
†
stands for the formal adjoint of $$\mathfrak {d}$$
d
on $$L^2((0,b),w\,\textrm{d}x)$$
L
2
(
(
0
,
b
)
,
w
d
x
)
. In the simpliest case $$k=1$$
k
=
1
, this symmetrization brings in the operator $$-\mathfrak {D}^2$$
-
D
2
, which can be seen as a ‘Laplacian’, and $$\mathfrak {D}f:=\mathfrak {D}_{\mathfrak {d}}f= \mathfrak {d}(f_{\text {even}})-\mathfrak {d}^\dagger (f_{\text {odd}})$$
D
f
:
=
D
d
f
=
d
(
f
even
)
-
d
†
(
f
odd
)
, a skew-symmetric operator in $$L^2(I,w\,\textrm{d}x)$$
L
2
(
I
,
w
d
x
)
, $$I=(-\,b,0)\cup (0,b)$$
I
=
(
-
b
,
0
)
∪
(
0
,
b
)
, is the symmetrization of $$\mathfrak {d}$$
d
. Investigated spectral properties include self-adjoint extensions, among them the Friedrichs extensions, of the symmetrized operators.