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 A239688 Number of paths joining opposite corners of an n X 2 grid with every vertex appearing at most twice in the path. 4
 2, 22, 218, 2100, 20701, 202664, 1990006, 19519656, 191538146, 1879225062, 18438413081, 180909222618, 1775009067937, 17415641736474, 170875080099492, 1676555203866592, 16449663882025188, 161397269556516646, 1583562991844385082, 15537262462108603226 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 LINKS Muhammad Kholilurrohman and Alois P. Heinz, Table of n, a(n) for n = 1..1000 (first 100 terms from Muhammad Kholilurrohman) FORMULA G.f.: -(8*x^10 +17*x^9 -38*x^8 -60*x^7 +74*x^6 +55*x^5 -56*x^4 -10*x^3 +6*x^2 +2*x) / (4*x^12 -20*x^11 -71*x^10 +116*x^9 +197*x^8 -268*x^7 -129*x^6 +252*x^5 -31*x^4 -80*x^3 +26*x^2 +8*x -1). - Alois P. Heinz, Nov 16 2014 EXAMPLE The 22 paths in a 2 X 2 grid whose nodes are numbered from 1 to 4 in the natural way are 124, 134, 12124, 12134, 12424, 12434, 13124, 13134, 13424, 13434, 1212434, 1213424, 1213434, 1242134, 1243124, 1243134, 1312424, 1312434, 1313424, 1342124, 1342134, 1343124. CROSSREFS Sequence in context: A255043 A308313 A304024 * A336894 A037567 A174200 Adjacent sequences:  A239685 A239686 A239687 * A239689 A239690 A239691 KEYWORD nonn AUTHOR Giovanni Resta, Mar 24 2014 EXTENSIONS a(21)-a(40) from Muhammad Kholilurrohman, Nov 16 2014 STATUS approved

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Last modified November 28 01:24 EST 2021. Contains 349396 sequences. (Running on oeis4.)