

A200000


Number of meanders filling out an n X n grid, reduced for symmetry.


4



1, 1, 0, 4, 42, 9050, 6965359, 26721852461, 429651752290375, 31194475941824888769, 9828395457980805457337560, 13684686862375136981850903785368, 83297108604256429529069019958551956425, 2226741508593975401942934273354241209226704830, 260577257822688861848154672171293101310412373160498171, 133631198381015786582155688877301469836628906260462969996612568, 299985729493560746632648983353916422875677601725131683097521792924081609
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OFFSET

1,4


COMMENTS

The sequence counts the distinct closed paths that visit every cell of an n X n square lattice at least once, that never cross any edge between adjacent squares more than once, and that do not selfintersect. Paths related by rotation and/or reflection of the square lattice are not considered distinct.
Are a(1) and a(2) the only two terms equal to 1? And is a(3) the only term equal to 0?  Daniel Forgues, Nov 24 2011
The answer is yes: There are several patterns that can straightforwardly be generalized to any grid of any size n>3, e.g., #13 and #6347 of the graphics for a(6) (resp. #24 or #28 of a(5) for odd n).  M. F. Hasler, Nov 24 2011


LINKS

Table of n, a(n) for n=1..17.
Dale Gerdemann, Video illustration for a(5) = 42
OEIS Wiki, Number of meanders filling out an nbyn grid (reduced for symmetry)
Jon Wild, Illustration for a(4) = 4
Jon Wild, Illustration for a(5) = 42
Jon Wild, Illustration for a(6) = 9050 [Warning: this is a large file!]
Du, Zhao Hui, C++ source code for A200000 and A200749


EXAMPLE

a(1) counts the paths that visit the single cell of the 1 X 1 lattice: there is one, the "fat dot".
The 4 solutions for n=4, 42 solutions for n=5 and 9050 solutions for n=6 are illustrated in the supporting .png files.


CROSSREFS

Cf. A200749 (version not reduced for symmetry).
Cf. A200893 (meanders on n X k rectangles instead of squares, reduced for symmetry).
Cf. A201145 (meanders on n X k rectangles, not reduced for symmetry).
Sequence in context: A134356 A156479 A111829 * A198209 A220774 A186678
Adjacent sequences: A199997 A199998 A199999 * A200001 A200002 A200003


KEYWORD

nonn,nice


AUTHOR

Jon Wild, Nov 20 2011


EXTENSIONS

a(8) and a(10) from Alex Chernov, May 28 2012
a(9) from Alex Chernov, added by Max Alekseyev, Jul 21 2013
a(11) to a(17) from Du, Zhao Hui, Apr 03 2014


STATUS

approved



