Now we equip a vector space VV with a notion of "size".
-
A norm is a function (
∥⋅∥:V→R
:
⋅
V
) such that the following properties hold (
,
x−∈V&y−∈V
x
y
x
V
y
V
and
,
α∈
α
α
):
-
∥x−∥≥0
x
0
with equality iff
x−=0
x
0
-
∥αx−∥=|α|⋅∥x−∥
α
x
α
⋅
x
-
∥x−+y−∥≤∥x−∥+∥y−∥
x
y
x
y
, (the triangle inequality).
In simple terms, the norm measures the size of a
vector. Adding the norm operation to a vector space yields
a normed vector space. Important example
include:
-
V=RN
V
N
,
∥
x
0
…
x
N
-
1
T∥≔∑
i
=0N−1
x
i
2≔x−Tx−
≔
x
0
…
x
N
-
1
i
0
N
1
x
i
2
x
x
-
V=CN
V
N
,
∥
x
0
…
x
N
-
1
T∥≔∑
i
=0N−1|
x
i
|2≔x−Hx−
≔
x
0
…
x
N
-
1
i
0
N
1
x
i
2
x
x
-
V=
l
p
V
l
p
,
∥xn∥≔∑n=−∞∞|xn|p1p
≔
x
n
n
x
n
p
1
p
-
V=
ℒ
p
V
ℒ
p
,
∥ft∥≔∫−∞∞|ft|pdt
1p
≔
f
t
t
f
t
p
1
p