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 A060972 Triangle giving numbers of closed plane meanders. 1
 1, 1, 1, 1, 6, 1, 1, 20, 20, 1, 1, 50, 160, 50, 1, 1, 105, 808, 808, 105, 1, 1, 196, 3066, 7294, 3066, 196, 1, 1, 336, 9552, 45588, 45588, 9552, 336, 1, 1, 540, 25740, 220362, 440172, 220362, 25740, 540, 1, 1, 825, 62040, 879840, 3133724, 3133724, 879840, 62040, 825, 1, 1, 1210, 136851, 3028454, 17752636, 31586346, 17752636, 3028454, 136851, 1210, 1 (list; table; graph; refs; listen; history; text; internal format)
 OFFSET 1,5 COMMENTS a(n) counts closed plane meanders according to the number of white regions when regions are colored black and white alternatively. So the sum of each row is given by A005315. The outer columns consist of 1's. The next-to-outer columns are given by A002415. This is also the number of arches above the x-axis going from an odd vertex to a higher even vertex(p) for closed plane meanders(M) with n arches. By symmetry, these same subsets exist for arches below the x-axis. For each meander solution, the total arches for the top and bottom that go from an odd vertex to a higher even vertex is n+1. Example: M(n,p): M(3,1)=1 [(top 16,23,45; bottom 12,34,56)], M(3,2)=6 [(top 14,23,56; bottom 16,25,34)(top 16,25,34; bottom 14,23,56) (top 12,36,45; bottom 16,25,34) (top 16,25,34; bottom 12,36,45) (top 12,36,45; bottom 14,23,56)(top 14,23,56; bottom 12,36,45)] M(3,3)=1 [(top 12,34,56; bottom 16,23,45)]. - Roger Ford, Sep 29 2014 LINKS Joerg Arndt and Andrew Howroyd, Table of n, a(n) for n = 1..210 (first 105 terms from Joerg Arndt) Reinhard O. W. Franz, and Berton A. Earnshaw, A constructive enumeration of meanders, Ann. Comb. 6 (2002), no. 1, 7-17. [Table 1 gives first 14 rows] EXAMPLE Triangle begins: 1; 1, 1; 1, 6, 1; 1, 20, 20, 1; 1, 50, 160, 50, 1; 1, 105, 808, 808, 105, 1; ... CROSSREFS Row sums give A005315, diagonals give A002415. Sequence in context: A146957 A146988 A203954 * A144066 A296827 A056941 Adjacent sequences:  A060969 A060970 A060971 * A060973 A060974 A060975 KEYWORD nonn,tabl AUTHOR F. Chapoton, May 09 2001; extended to 14 rows, Jul 31 2011 STATUS approved

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Last modified January 18 00:56 EST 2021. Contains 340249 sequences. (Running on oeis4.)