

A086441


Number of inequivalent ways a semiinfinite curve can cross a straight line n times.


2



1, 1, 2, 4, 11, 27, 79, 213, 644, 1840, 5660
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OFFSET

1,3


COMMENTS

This uses a too broad notion of equivalence. Besides the obvious reflection in a plane perpendicular to the straight line, if the end of the curve is in a free region of the plane, it is extended to infinity and the direction of the curve can then be reversed. A000560 uses a better definition of equivalence.


LINKS

Table of n, a(n) for n=1..11.
Index entries for sequences obtained by enumerating foldings


EXAMPLE

The a(3) = 2 solutions with 3 crossings. The line is drawn horizontally. The curve starts at oo and ends at X. The crossings are indicated by stars.
..............X
...../..\....../
***
.../......\../
../........
./
oo
........
......./...\
....../..X..\
...../......\
***
.../......../
../.......
./
oo


CROSSREFS

Isomorphism classes (using too generous a definition of isomorphism) from A000682. Cf. A000560, A001011.
Sequence in context: A071314 A309886 A123441 * A316696 A319616 A148130
Adjacent sequences: A086438 A086439 A086440 * A086442 A086443 A086444


KEYWORD

nonn,more


AUTHOR

N. J. A. Sloane, Sep 09 2003


STATUS

approved



