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A006215 Number of down-up permutations of n+6 starting with n+1.

%I A006215 M5007 #31 Jul 08 2025 16:43:38
%S A006215 0,16,122,800,5296,36976,275792,2204480,18870016,172585936,1681843712,
%T A006215 17411416160,190939611136,2211961358896,26999750469632,
%U A006215 346419349043840,4661658528710656,65657186909139856,966054350401175552,14822897275566895520
%N A006215 Number of down-up permutations of n+6 starting with n+1.
%C A006215 Entringer numbers.
%D A006215 R. C. Entringer, A combinatorial interpretation of the Euler and Bernoulli numbers, Nieuw Archief voor Wiskunde, 14 (1966), 241-246.
%D A006215 N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
%H A006215 B. Bauslaugh and F. Ruskey, <a href="http://dx.doi.org/10.1007/BF01932127">Generating alternating permutations lexicographically</a>, Nordisk Tidskr. Informationsbehandling (BIT) 30 16-26 1990.
%H A006215 J. Millar, N. J. A. Sloane and N. E. Young, A new operation on sequences: the Boustrophedon transform, J. Combin. Theory, 17A (1996) 44-54 (<a href="http://neilsloane.com/doc/bous.txt">Abstract</a>, <a href="http://neilsloane.com/doc/bous.pdf">pdf</a>, <a href="http://neilsloane.com/doc/bous.ps">ps</a>).
%H A006215 C. Poupard, <a href="http://dx.doi.org/10.1016/0012-365X(82)90293-X">De nouvelles significations énumeratives des nombres d'Entringer</a>, Discrete Math., 38 (1982), 265-271.
%H A006215 <a href="/index/To#tournament">Index entries for sequences related to tournaments</a>
%F A006215 From _Emeric Deutsch_, May 15 2004: (Start)
%F A006215 a(n) = Sum_{i=0..floor((n-1)/2)} (-1)^i * binomial(n, 2*i+1) * E[n + 4 - 2*i], where E[j] = A000111(j) = j! * [x^j] (sec(x) + tan(x)) are the up/down or Euler numbers.
%F A006215 a(n) = T(n+5, n), where T is the triangle in A008282. (End)
%e A006215 a(1)=16 because we have 2143657, 2143756, 2153647, 2153746, 2154637, 2154736, 2163547, 2163745, 2164537, 2164735, 2165734, 2173546, 2173645, 2174536, 2174635 and 2175634.
%p A006215 f:=sec(x)+tan(x): fser:=series(f,x=0,30): E[0]:=1: for n from 1 to 25 do E[n]:=n!*coeff(fser,x^n) od: a:=n->sum((-1)^i*binomial(n,2*i+1)*E[n+4-2*i],i=0..floor((n-1)/2)): seq(a(n),n=0..15);
%p A006215 # Alternatively after _Alois P. Heinz_ in A000111:
%p A006215 b := proc(u, o) option remember;
%p A006215 `if`(u + o = 0, 1, add(b(o - 1 + j, u - j), j = 1..u)) end:
%p A006215 a := n -> b(n, 5): seq(a(n), n = 0..21); # _Peter Luschny_, Oct 27 2017
%t A006215 t[n_, 0] := If[n == 0, 1, 0]; t[n_ , k_ ] := t[n, k] = t[n, k - 1] + t[n - 1, n - k]; a[n_] := t[n + 5, n]; Array[a, 30, 0] (* _Jean-François Alcover_, Feb 12 2016 *)
%Y A006215 Cf. A000111, A008282.
%K A006215 nonn,easy
%O A006215 0,2
%A A006215 _N. J. A. Sloane_
%E A006215 More terms from _Jean-François Alcover_, Feb 12 2016