A026385 Sum{T(n-k,k)}, 0<=k<=[ n/2 ], where T is the array in A026374.

1, 1, 2, 4, 6, 11, 20, 33, 59, 104, 178, 314, 549, 952, 1669, 2913, 5074, 8872, 15482, 27007, 47172, 82325, 143675, 250848, 437822, 764198, 1334041, 2328512, 4064457, 7094833, 12384034, 21616716, 37732990, 65863651

Offset: 0

Author

Clark Kimberling

Keywords

• nonn

Links

• Vincenzo Librandi, Table of n, a(n) for n = 0..1000
• Index entries for linear recurrences with constant coefficients, signature (0,1,3,1).

Programs

Magma

I:=[1,1,2,4]; [n le 4 select I[n] else Self(n-2)+3*Self(n-3)+Self(n-4): n in [1..40]]; // Vincenzo Librandi, Jun 19 2014

Mathematica

LinearRecurrence[{0,1,3,1},{1,1,2,4},40] (* Harvey P. Dale, Jun 18 2014 *)
CoefficientList[Series[(1 + x + x^2)/(1 - x^2 - 3 x^3 - x^4), {x, 0, 40}], x] (* Vincenzo Librandi, Jun 19 2014 *)

Formula

G.f.: (1+x+x^2)/(1-x^2-3x^3-x^4). - Ralf Stephan, Apr 30 2004

a(n) = a(n-2) + 3*a(n-3) + a(n-4) for n>3. - Vincenzo Librandi, Jun 19 2014