A121812 a(n) = a(n-4)*a(n-3) + a(n-4)*a(n-2) + a(n-3)*a(n-2) + a(n-4)*a(n-1) + a(n-3)*a(n-1) + a(n-2)*a(n-1).

0, 1, 1, 1, 3, 12, 67, 1123, 93127, 112028175, 10566272277846, 1184716081561127371921, 12518165521628430511804483358652025, 14830472137388987962469618424964700332186681194373079941

Offset

1,5

Links

Harvey P. Dale, Table of n, a(n) for n = 1..20

Formula

a(n) = sum_{j=2..4} sum_{1<=k<j} a(n-j)*a(n-k). [Oct 14 2009]

Mathematica

a[0] = 0; a[1] = 1; a[2] = 1; a[3] = 1; a[n_] : a[n] = a[ -4 + n] a[ -3 + n] + a[ -4 + n] a[ -2 + n] + a[ -3 + n] a[ -2 + n] + a[ -4 + n] a[ -1 + n] + a[ -3 + n] a[ -1 + n] + a[ -2 + n] a[ -1 + n] b = Table[a[n], {n, 0, 15}]
nxt[{a_, b_, c_, d_}]:={b, c, d, Total[Times@@@Subsets[{a, b, c, d}, {2}]]}; NestList[nxt, {0, 1, 1, 1}, 20][[;; , 1]] (* Harvey P. Dale, Sep 26 2023 *)

Crossrefs

Sequence in context: A107103 A107887 A187118 * A039750 A365655 A366228

Adjacent sequences: A121809 A121810 A121811 * A121813 A121814 A121815

Keyword

nonn

Author

Roger L. Bagula, Aug 30 2006

Extensions

Definition replaced by recurrence - The Assoc. Editors of the OEIS, Oct 14 2009

Status

approved