A121813 a(n) = sum_{j=1..4} product_{k=1..4, k<>j} a(n-k).

0, 1, 1, 1, 1, 4, 13, 121, 8401, 17724001, 19980403610080, 3017939125082738100693961, 1069257489122187637992525695378883464262898201

Offset

1,6

Links

Table of n, a(n) for n=1..13.

Formula

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

Example

a(8) = a(4)*a(5)*a(6)+a(4)*a(5)*a(7)+a(4)*a(6)*a(7)+a(5)*a(6)*a(7) = 1*1*4 +1*1*13 +1*4*13 +1*4*13 = 121.

Mathematica

a[0] = 0; a[1] = 1; a[2] = 1; a[3] = 1; a[n_] : a[n] = a[ -4 + n] a[ -3 + n] a[ -2 + n] + a[ -4 + n] a[ -3 + n] a[ -1 + n] + a[ -4 + n] a[ -2 + n] a[ -1 + n] + a[ -3 + n] a[ -2 + n] a[ -1 + n] b = Table[a[n], {n, 0, 15}]
RecurrenceTable[{a[0]==0, a[1]==a[2]==a[3]==1, a[n]==a[n-4]a[n-3]a[n-2]+a[n-4]a[n-3]a[n-1]+a[n-4]a[n-2]a[n-1]+a[n-3]a[n-2]a[n-1]}, a, {n, 15}] (* Harvey P. Dale, Aug 18 2014 *)

Crossrefs

Sequence in context: A276912 A045886 A015460 * A197897 A203380 A006104

Adjacent sequences: A121810 A121811 A121812 * A121814 A121815 A121816

Keyword

nonn,easy

Author

Roger L. Bagula, Aug 30 2006

Extensions

Explicit definition provided by the Assoc. Eds. of the OEIS - Mar 27 2010

Status

approved