X ( 2 , 15 , 3 , 16 ) X ( 4 , 18 , 5 , 17 ) X ( 6 , 7 , 1 , 8 ) X ( 8 , 1 , 9 , 2 ) X ( 10 , 4 , 11 , 3 ) X ( 12 , 5 , 13 , 6 ) X ( 14 , 10 , 15 , 9 ) X ( 16 , 11 , 17 , 12 ) X ( 18 , 14 , 7 , 13 ) {\displaystyle X(2,15,3,16)X(4,18,5,17)X(6,7,1,8)X(8,1,9,2)X(10,4,11,3)X(12,5,13,6)X(14,10,15,9)X(16,11,17,12)X(18,14,7,13)}
