A005982 3 up, 3 down, 3 up, ... permutations of length 3n+1. (Formerly M5107)

1, 20, 1301, 202840, 61889101, 32676403052, 27418828825961, 34361404413755056, 61335081309931829401, 150221740688275657957940, 489799709605132718770274141, 2073641570051429601078643837960, 11163099186064084100687107863253381

Offset

1,2

References

N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).

P. R. Stein, personal communication.

Links

Alois P. Heinz, Table of n, a(n) for n = 1..100

P. R. Stein & N. J. A. Sloane, Correspondence, 1975

Maple

b:= proc(u, o, t) option remember; `if`(u+o=0, 1, add(`if`(t=3,
       b(o-j, u+j-1, 1), b(u+j-1, o-j, t+1)), j=1..o))
    end:
a:= n-> b(0, 3*n+1, 0):
seq(a(n), n=1..20);  # Alois P. Heinz, Oct 06 2013

Mathematica

b[u_, o_, t_] := b[u, o, t] = If[u+o == 0, 1, Sum[If[t == 3, b[o-j, u+j-1, 1], b[u+j-1, o-j, t+1]], {j, 1, o}]] ; a[n_] := b[0, 3*n+1, 0]; Table[a[n], {n, 1, 20}] (* Jean-François Alcover, Nov 25 2014, after Alois P. Heinz *)

Crossrefs

Cf. A229884.

Sequence in context: A160253 A338961 A274645 * A229476 A177597 A127847

Adjacent sequences: A005979 A005980 A005981 * A005983 A005984 A005985

Keyword

nonn

Author

N. J. A. Sloane.

Extensions

Typo in name fixed by Alois P. Heinz, Oct 06 2013

Status

approved