kk
k
c c c
k
k k
c c c c k k
k k
r
r
n
n
n
nn Euclidean space
n
CB CA AB a
b c a b c
a b c
a b c
A B
A
B
x s x t x s t
x s x t x s t
n
x r x s x t x r s t x r s t
x s x t x x s t x s t
k
k n
A B
CAB
AC AB AC AB AC
A A B C
ABC
W V V
W V W
V W
W V
pS
qTi
j i j
n
a a b b
a a b b
k k
a a a m m
b b b
V V
n n
n