A168199 a(n) = 3*n - a(n-1) + 1 with n > 1, a(1)=2.

2, 5, 5, 8, 8, 11, 11, 14, 14, 17, 17, 20, 20, 23, 23, 26, 26, 29, 29, 32, 32, 35, 35, 38, 38, 41, 41, 44, 44, 47, 47, 50, 50, 53, 53, 56, 56, 59, 59, 62, 62, 65, 65, 68, 68, 71, 71, 74, 74, 77, 77, 80, 80, 83, 83, 86, 86, 89, 89, 92, 92, 95, 95, 98, 98, 101, 101, 104, 104, 107

Offset: 1

Author

Vincenzo Librandi, Nov 20 2009

Keywords

• nonn
• easy

Links

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

Programs

Magma

I:=[2, 5, 5]; [n le 3 select I[n] else Self(n-1)+Self(n-2)-Self(n-3): n in [1..40]]; // Vincenzo Librandi, Feb 28 2012

Mathematica

LinearRecurrence[{1, 1, -1}, {2, 5, 5}, 70] (* Vincenzo Librandi, Feb 28 2012 *)

Formula

a(n) = (6*n + 5 + 3*(-1)^n)/4. - Jon E. Schoenfield, Jun 24 2010

G.f.: x*(2-x)*(1+2*x)/((1+x)*(1-x)^2). - Bruno Berselli, Feb 28 2012