Definition 4.5
(
U
2
-norm)
.
Given
f
:
G
→
ℂ
, define its
U
2
-norm
by the formula
∥
f
∥
U
2
(
G
)
4
=
𝔼
x
,
a
,
b
∈
G
f
(
x
)
f
(
x
+
a
)
f
(
x
+
b
)
¯
f
(
x
+
a
+
b
)
.