A005982 3 up, 3 down, 3 up, ... permutations of length 3n+1.

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

Offset: 1

Author

N. J. A. Sloane

Keywords

• nonn

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

Crossrefs

Cf. A229884.

Programs

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 *)

Extensions

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