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 A337581 a(n) is the number of semi-meanders with n top arches that have both an arch of length 1 adjacent to the center of the top arch configuration and an arch of length 1 starting or ending the top arch configuration. 0
 1, 2, 4, 8, 16, 40, 96, 264, 696, 2016, 5624, 16840, 48792, 149512, 445112, 1387384, 4215496, 13312752, 41097864, 131146520, 410045672, 1319612232, 4169110888, 13511677040, 43060097728, 140383359392 (list; graph; refs; listen; history; text; internal format)
 OFFSET 2,2 COMMENTS The number of semi-meanders with n top arches is A000682(n). The number of semi-meanders with n top arches that have a starting or ending arch of length 1 is 2*A000682(n-1). The number of semi-meanders with n top arches that have an arch adjacent to the center with length 1 is 2*A000682(n-1). LINKS Table of n, a(n) for n=2..27. FORMULA For n >= 4: a(n) = 4*A000682(n-2). EXAMPLE For n = 5: (10) is a starting or ending arch of length 1, [10] is an arch adjacent to the center with length 1, a(5) = 8. (10)101[10]100, 110[10]010(10), (10)1[10]11000, 11100[10]0(10), (10)111[10]000, 111[10]000(10), (10)1[10]01100, 11001[10]0(10). /\ = arch with length 1, (10) = arches with length 1 at ends or adjacent to center /\ /\ /\ / \ / \ / \ / /\ \ / /\ \ /\ / \ 1 10 0 1(10) 0(10) /\ /\ / /\ /\\ center| end (10) 10 1(10)100 end center| CROSSREFS Cf. A000682. Sequence in context: A028497 A197244 A350495 * A259377 A275071 A018575 Adjacent sequences: A337578 A337579 A337580 * A337582 A337583 A337584 KEYWORD nonn,more AUTHOR Roger Ford, Sep 01 2020 STATUS approved

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Last modified September 27 20:41 EDT 2023. Contains 365714 sequences. (Running on oeis4.)