.. 2 1 3 3 1 2 2 3 N N 1 N N(...
2
...
2 1
3
3
1 2
2 3
n n 1
n
n
(III) VP =1
1
1
1
2
...
2 1
3
3
1 2
2 3
n n 1
n
n
(III) VP =1
1
1
1