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A008909
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Join 2n points on a line with n arcs above the line; form graph with the arcs as nodes, joining 2 nodes when the arcs cross. a(n) is the number of cases in which the graph is a path.
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0
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1, 1, 3, 8, 21, 56, 153, 428, 1222, 3549, 10454, 31159, 93801, 284788, 871007, 2681018
(list; graph; refs; listen; history; internal format)
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OFFSET
| 1,3
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COMMENTS
| a(n+1) = number of (finite) positive integer sequences b(1),..,b(k) with b(1)+sum(1+max{b(i+1)-b(i), 0}, i=1..k-1) <= n. - Klaus Strassburger. E.g. a(4)=8 since the integer sequences are 1; 2; 3; 1,1; 1,2; 2,1; 2,2; 1,1,1.
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CROSSREFS
| Sequence in context: A128105 A085560 A094374 * A006835 A014318 A158495
Adjacent sequences: A008906 A008907 A008908 * A008910 A008911 A008912
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KEYWORD
| nonn
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AUTHOR
| klazar(AT)kam.mff.cuni.cz (Martin Klazar)
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EXTENSIONS
| More terms from Klaus Strassburger (strass(AT)ddfi.uni-duesseldorf.de), Sep 24 2001
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