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A086031
Number of nonisomorphic ways a loop can cross two parallel roads 2n times.
2
1, 1, 3, 7, 37, 207, 1470, 11169, 91166, 776841, 6865598, 62443973
OFFSET
0,3
COMMENTS
There is no obligation to cross the lower road (cf. A077054).
CROSSREFS
Cf. A077054. Isomorphism classes from A085973.
Sequence in context: A087208 A161675 A208809 * A174734 A152560 A162926
KEYWORD
nonn
AUTHOR
N. J. A. Sloane and Jon Wild, Aug 25 2003
STATUS
approved