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
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