A287809 Number of septenary sequences of length n such that no two consecutive terms have distance 2.

1, 7, 39, 219, 1231, 6921, 38913, 218789, 1230147, 6916539, 38888455, 218651553, 1229375193, 6912200477, 38864063403, 218514412227, 1228604118319, 6907865088537, 38839687552689, 218377358251349, 1227833528067027, 6903532420748427, 38815326992539159

Offset: 0

Author

David Nacin, Jun 01 2017

Keywords

• nonn
• easy

Examples

			For n=2 the a(2)=49-10=39 sequences contain every combination except these ten: 02,20,13,31,24,42,35,53,46,64.
		

Links

• Index entries for linear recurrences with constant coefficients, signature (6, 0, -13, 6).

Crossrefs

Cf. A040000, A003945, A083318, A078057, A003946, A126358, A003946, A055099, A003947, A015448, A126473. A287804-A287819.

Programs

Mathematica

LinearRecurrence[{6, 0, -13, 6}, {1, 7, 39, 219, 1231}, 40]

Python

def a(n):
 if n in [0, 1, 2, 3, 4]:
  return [1, 7, 39, 219, 1231][n]
 return 6*a(n-1)-13*a(n-3)+6*a(n-4)

Formula

For n>4, a(n) = 6*a(n-1) - 13*a(n-3) + 6*a(n-4), a(1)=7, a(2)=39, a(3)=219, a(4)=1231.

G.f.: (1 + x - 3*x^2 - 2*x^3 + 2*x^4)/(1 - 6*x + 13*x^3 - 6*x^4).