login
A005982
3 up, 3 down, 3 up, ... permutations of length 3n+1.
(Formerly M5107)
2
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
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
KEYWORD
nonn
AUTHOR
EXTENSIONS
Typo in name fixed by Alois P. Heinz, Oct 06 2013
STATUS
approved