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 A200000 Number of meanders filling out an n X n grid, reduced for symmetry. 5
 1, 1, 0, 4, 42, 9050, 6965359, 26721852461, 429651752290375, 31194475941824888769, 9828395457980805457337560, 13684686862375136981850903785368, 83297108604256429529069019958551956425, 2226741508593975401942934273354241209226704830, 260577257822688861848154672171293101310412373160498171, 133631198381015786582155688877301469836628906260462969996612568, 299985729493560746632648983353916422875677601725131683097521792924081609 (list; graph; refs; listen; history; text; internal format)
 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 self-intersect. 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 n-by-n 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!] Zhao Hui Du, 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: A355130 A355124 A111829 * A198209 A220774 A296683 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 Zhao Hui Du, Apr 03 2014 STATUS approved

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Last modified June 4 20:02 EDT 2023. Contains 363128 sequences. (Running on oeis4.)