Consider the transformation of the data vector
xx:
y=Qx
y
Q
x
y0y1⋮yN-1=(
q00q01…q0N−1
q10q11…q1N−1
⋮⋮⋮⋮
qN−10qN−11…qN−1N−1
)x0x1⋮xN-1
y0
y1
⋮
yN-1
q
0
0
q
0
1
…
q
0
N
1
q
1
0
q
1
1
…
q
1
N
1
⋮
⋮
⋮
⋮
q
N
1
0
q
N
1
1
…
q
N
1
N
1
x0
x1
⋮
xN-1
yk=∑nqknxn
y
k
n
q
k
n
x
n
If QQ is an orthogonal
matrix (meaning that
QTQ=I
Q
Q
I
)
then the data vector
xx
can be recovered from yy
using the transpose of QQ:
x=QTy
x
Q
y
x0x1⋮xN-1=(
q00q10…qN−10
q01q11…qN−11
⋮⋮⋮⋮
q0N−1q1N−1…qN−1N−1
)y0y1⋮yN-1
x0
x1
⋮
xN-1
q
0
0
q
1
0
…
q
N
1
0
q
0
1
q
1
1
…
q
N
1
1
⋮
⋮
⋮
⋮
q
0
N
1
q
1
N
1
…
q
N
1
N
1
y0
y1
⋮
yN-1
xn=∑kqknyk
x
n
k
q
k
n
y
k