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A077550 Number of nonisomorphic ways a river (or undirected line) can cross two perpendicular roads n times (orbits of A076875 under symmetry group of order 8). 1
1, 1, 2, 3, 9, 21, 54, 131 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,3

COMMENTS

There is no constraint on touching any particular sector.

LINKS

Table of n, a(n) for n=0..7.

CROSSREFS

Cf. A076875 (total number of solutions), A076906, A076907 (if line is directed)

Sequence in context: A015951 A101531 A099607 * A056780 A141505 A111360

Adjacent sequences:  A077547 A077548 A077549 * A077551 A077552 A077553

KEYWORD

nonn

AUTHOR

N. J. A. Sloane and Jon Wild, Aug 25 2003

STATUS

approved

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Last modified April 24 15:35 EDT 2014. Contains 240985 sequences.