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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 (list; graph; refs; listen; history; text; internal format)
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-Fran├žois Alcover, Nov 25 2014, after Alois P. Heinz *)

CROSSREFS

Cf. A229884.

Sequence in context: A153469 A160253 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

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Last modified January 18 23:05 EST 2019. Contains 319282 sequences. (Running on oeis4.)