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例子
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a(4)=1,因为(q_1,q_2,q_3,q_4)=(2,5,3,7)是唯一合适的置换。
a(5)=3,因为正好有三个合适的置换(q_1,q_2,q_3,q_4,q_5):(2,3,7,5,11),(2,5,7,3,11)和(2,7,3,5,11)。
a(6)=5,因为正好有五个合适的置换(q_1,q_2,q_3,q_4,q_5,q_6):(2,5,3,11,7,17,13),(2,5,7,11,13,13),(2,7,5,11,3,13),(2,7,11,5,3,13),(2,11,5,7,3,13)。
a(7)=10,十个合适的排列(q_1,…,q_7)如下:
(2,3,13,5,7,11,17), (2,7,3,13,11,5,17), (2,7,5,11,3,13,17),
(2,7,11,5,13,3,17), (2,11,3,13,7,5,17), (2,11,7,5,13,3,17),
(2,11,7,13,3,5,17), (2,11,7,13,5,3,17), (2,13,3,11,7,5,17),
(2,13,7,11,3,5,17).
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