A139806 a(n)=a(n-1)+a(n-2)+a(n-3)+2a(n-4), a(0)=1, a(1)=3, a(2)=7, a(3)=15.

1, 3, 7, 15, 27, 55, 111, 223, 443, 887, 1775, 3551, 7099, 14199, 28399, 56799, 113595, 227191, 454383, 908767, 1817531, 3635063, 7270127, 14540255, 29080507, 58161015, 116322031, 232644063, 465288123, 930576247, 1861152495, 3722304991, 7444609979, 14889219959

Offset: 0

Author

Paul Curtz, May 23 2008

Keywords

• nonn
• easy

Links

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

Formula

G.f. ( -1-2*x-3*x^2-4*x^3 ) / ( (2*x-1)*(1+x)*(x^2+1) ). - R. J. Mathar, Feb 06 2011