A287811 Number of septenary sequences of length n such that no two consecutive terms have distance 5.

1, 7, 45, 291, 1881, 12159, 78597, 508059, 3284145, 21229047, 137226717, 887047443, 5733964809, 37064931183, 239591481525, 1548743682699, 10011236540769, 64713650292711, 418315611378573, 2704034619149571, 17479154549033145, 112987031151647583

Offset: 0

Author

David Nacin, Jun 01 2017

Keywords

• nonn
• easy

Examples

			For n=2 the a(2) = 49-4 = 45 sequences contain every combination except these four: 05, 50, 16, 61.
		

Links

• Index entries for linear recurrences with constant coefficients, signature (6, 3).

Crossrefs

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

Cf. A287804-A287819.

Programs

Mathematica

LinearRecurrence[{6, 3}, {1,7}, 40]

Python

def a(n):
 if n in [0, 1]:
  return [1, 7][n]
 return 6*a(n-1)-3*a(n-2)

Formula

a(n) = 6*a(n-1) + 3*a(n-2), a(0)=1, a(1)=7.

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

a(n) = A090018(n-1)+A090018(n). - R. J. Mathar, Oct 20 2019