For $$1\le p<\infty $$
1
≤
p
<
∞
, we prove that the dense subspace $$\mathcal {Y}_p$$
Y
p
of $$\ell _p(\Gamma )$$
ℓ
p
(
Γ
)
comprising all elements y such that $$y \in \ell _q(\Gamma )$$
y
∈
ℓ
q
(
Γ
)
for some $$q \in (0,p)$$
q
∈
(
0
,
p
)
admits a $$C^{\infty }$$
C
∞
-smooth norm which locally depends on finitely many coordinates. Moreover, such a norm can be chosen as to approximate the $$\left\| \cdot \right\| _p$$
·
p
-norm. This provides examples of dense subspaces of $$\ell _p(\Gamma )$$
ℓ
p
(
Γ
)
with a smooth norm which have the maximal possible linear dimension and are not obtained as the linear span of a biorthogonal system. Moreover, when $$p>1$$
p
>
1
or $$\Gamma $$
Γ
is countable, such subspaces additionally contain dense operator ranges; on the other hand, no non-separable operator range in $$\ell _1(\Gamma )$$
ℓ
1
(
Γ
)
admits a $$C^1$$
C
1
-smooth norm.