Search: id:A077460
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%I A077460
%S A077460 1,1,1,3,12,70,464,3482,27779,233556
%N A077460 Number of nonisomorphic ways a loop can cross a road (running East-West) 
               2n times.
%C A077460 Nonisomorphic closed meanders, where two closed meanders are considered 
               equivalent if one can be obtained from the other by reflections in 
               an East-West or North-South mirror (a group of order 4).
%e A077460 A meander can be specified by marking 2n equally spaced points along 
               a line and recording the order in which the meander visits the points.
%e A077460 For n = 2, 4, 6, 8 the solutions are as follows:
%e A077460 n=2: 1 2
%e A077460 n=4: 1 2 3 4
%e A077460 n=6: 1 2 3 4 5 6, 1 2 3 6 5 4, 1 2 5 4 3 6
%e A077460 n=8: 1 2 3 4 5 6 7 8, 1 2 3 4 5 8 7 6, 1 2 3 4 7 6 5 8, 1 2 7 6 3 4 5 
               8, 1 2 3 6 7 8 5 4, 1 2 3 6 5 4 7 8, 1 2 7 6 5 4 3 8, 1 2 3 8 5 6 
               7 4, 1 2 3 8 7 4 5 6, 1 2 5 6 7 4 3 8, 1 2 7 4 5 6 3 8, 1 4 3 2 7 
               6 5 8
%Y A077460 The total number of closed meanders with 2n crossings is given in A005315. 
               Cf. A077055, A078104, A078105, A078591.
%Y A077460 Sequence in context: A102078 A113341 A125862 this_sequence A001205 A112320 
               A103366
%Y A077460 Adjacent sequences: A077457 A077458 A077459 this_sequence A077461 A077462 
               A077463
%K A077460 nonn,nice
%O A077460 0,4
%A A077460 N. J. A. Sloane (njas(AT)research.att.com) and Jon Wild (wild(AT)music.mcgill.ca), 
               Dec 03 2002

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