Corollary 3.5
(Generalised parallelogram law)
.
Assuming that:
X
=
{
x
1
,
…
,
x
n
}
non-spherical
Then
there exists
c
1
,
…
,
c
n
not all
0
with
∑
c
i
=
0
and there exists
b
≠
0
such that
∑
i
c
i
∥
x
i
∥
2
=
b
.