We study the class of q-Fourier multiplier operators
T
m
:
=
F
q
(
F
q
)
, which are acted on the q-Sobolev space
H
*
,
q
s
(
ℝ
q
)
, and we obtain the exact expression and some properties for the extremal functions of the best approximation problem in quantum calculus
inf
f
∈
H
*
,
q
s
(
ℝ
q
)
{
η
∥
f
∥
H
*
,
q
s
(
ℝ
q
)
2
+
∥
g
−
T
m
f
∥
L
2
(
ℝ
q
,
+
)
2
}
, where
η
>
0
and
g
∈
L
2
(
ℝ
q
,
+
)
. As an application, we provide numerical approximate formulas for a limit case
η
↑
0
; using q-calculus, which generalizes the Gauss-Kronrod method studied given in [14] in one-dimensional space.