a(1)=1到a(5)=20排列:
(1) (1,2) (1,2,3) (1,2,3,4) (1,2,3,4,5)
(1,3,2) (1,3,2,4) (1,2,3,5,4)
(1,4,2,3) (1,2,4,3,5)
(1,4,3,2) (1,2,4,5,3)
(1,2,5,4,3)
(1,3,2,5,4)
(1,3,4,2,5)
(1,3,4,5,2)
(1,3,5,2,4)
(1,3,5,4,2)
(1,4,2,3,5)
(1,4,2,5,3)
(1,4,3,2,5)
(1,4,5,2,3)
(1,4,5,3,2)
(1,5,2,3,4)
(1,5,2,4,3)
(1,5,3,2,4)
(1,5,3,4,2)
(1,5,4,3,2)