Consider the level-1 subspace and its orthonormal basis:
V
1
=span
φ
1
,
n
t
n∈Z
V
1
span
φ
1
,
n
t
n
(1)
φ
1
,
n
t=12φ12t−n
φ
1
,
n
t
1
2
φ
1
2
t
n
(2)
Since
V1⊂V0
V1
V0
(
i.e.,
V
0
V
0
is more detailed than
V
1
V
1
) and since
φ
1
,
0
t∈
V
0
φ
1
,
0
t
V
0
, there must exist coefficients
hn
n∈Z
h
n
n
such that
φ
1
,
0
t=∑n=−∞∞hn
φ
0
,
n
t
φ
1
,
0
t
n
h
n
φ
0
,
n
t
(3)
⇔
12φ12t=∑n=−∞∞hnφt−n
⇔
1
2
φ
1
2
t
n
h
n
φ
t
n
(4)
φt=2∑n=−∞∞hnφ2t−n
φ
t
2
n
h
n
φ
2
t
n
(5)
To be a valid scaling function,
φt
φ
t
must obey the scaling equation for some coefficient
set
hn
h
n
.