“…When (x 1 , x 2 ) � (a 6 , a 5 ), then successively we get lk(a 2 ) � C 6 (a 1 , a 3 , a 6 , a 5 , 3, 2), lk(a 5 ) � C 6 (a 1 , a 4 , a 6 , a 2 , 3, 1), lk(a 4 ) � C 6 (a 3 , a 1 , a 5 , a 6 , 5, 4), lk(a 3 ) � C 6 (a 4 , a 1 , a 2 , a 6 , 6, 4), lk(a 6 ) � C 6 (a 3 , a 2 , a 5 , a 4 , 5, 6). Considering lk(1), it is easy to see that (n 2 , n 3 , n 4 ) ∈{(4, 5, 6), (4,6,5), (5,4,6), (5,6,4), (6,4,5), (6,5,4) a 1 , a 2 ) (a 4 , a 6 ). So, we search for (n 2 , n 3 , n 4 ) ∈ {(4, 5, 6), (4,6,5), (5,4,6)}.…”