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EXAMPLE
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a(1) = 1 due to the circular permutation (0,1).
a(2) = 1 due to the circular permutation (0,2,1).
a(3) = 1 due to the circular permutation (0,3,2,1).
a(5) = 2 due to the circular permutations
(0,3,2,4,5,1) and (0,3,5,4,2,1).
a(6) = 3 due to the circular permutations
(0,3,6,5,4,2,1), (0,6,3,2,4,5,1), (0,6,3,5,4,2,1).
a(7) = 7 due to the circular permutations
(0,3,6,5,4,2,7,1), (0,3,6,5,4,7,2,1), (0,6,3,2,4,7,5,1),
(0,6,3,2,5,4,7,1), (0,6,3,2,7,4,5,1), (0,6,3,5,4,2,7,1),
(0,6,3,5,4,7,2,1).
a(8) = 11 due to the circular permutations
(0,3,6,5,8,4,2,7,1), (0,3,6,5,8,4,7,2,1),
(0,3,6,8,4,2,7,5,1), (0,4,6,8,4,7,2,5,1),
(0,3,6,8,5,4,2,7,1), (0,3,6,8,5,4,7,2,1),
(0,6,3,2,4,7,5,8,1), (0,6,3,2,5,8,4,7,1),
(0,6,3,2,7,4,5,8,1), (0,6,3,5,8,4,2,7,1),
(0,6,3,5,8,4,7,2,1).
a(9) = 9 due to the circular permutations
(0,6,3,9,2,4,7,5,8,1), (0,6,3,9,2,5,8,4,7,1),
(0,6,3,9,2,7,4,5,8,1), (0,6,3,9,5,8,4,2,7,1),
(0,6,3,9,5,8,4,7,2,1), (0,6,3,9,8,4,2,7,5,1),
(0,6,3,9,8,4,7,2,5,1), (0,6,3,9,8,5,4,2,7,1),
(0,6,3,9,8,5,4,7,2,1).
a(20) > 0 due to the circular permutation
(0,3,12,9,15,18,6,20,19,14,13,4,2,7,16,17,11,10,5,8,1).
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