|
EXAMPLE
|
a(0) = 1: [].
a(1) = 1: [2].
a(2) = 9: [2,1], [2,3], [2,4], [3,1], [3,2], [3,4], [4,1], [4,2], [4,3].
a(3) = 70: [2,3,1], [2,3,2], [2,3,4], ..., [6,5,2], [6,5,3], [6,5,4].
a(4) = 581: [2,3,2,1], [2,3,4,1], [2,3,4,2], ..., [8,7,6,3], [8,7,6,4], [8,7,6,5].
|