A140295 a(n) = a(n-1) + a(n-2) + 2a(n-3).

1, -2, 4, 4, 4, 16, 28, 52, 112, 220, 436, 880, 1756, 3508, 7024, 14044, 28084, 56176, 112348, 224692, 449392, 898780, 1797556, 3595120, 7190236, 14380468, 28760944, 57521884, 115043764, 230087536, 460175068, 920350132, 1840700272, 3681400540

Offset: 0

Author

Paul Curtz, May 25 2008

Keywords

• sign
• easy

Links

• Index entries for linear recurrences with constant coefficients, signature (1,1,2).

Formula

a(n+1) - 2a(n) = period 3: repeat -4, 8, -4.

G.f.: (1-3x+5x^2)/((1-2x)(1+x+x^2)). a(n)=(4*k(n)+3*2^n)/7 where k(n) is the 3-period sequence 1,-5,4,... reminiscent to A130815. [From R. J. Mathar, Oct 30 2008]