G.f. A(x) satisfies: A(x) = (1 - x) / (1 - 2 * x - x^2 - x^3 * A(x)).

Terms

a(0) =1a(1) =1a(2) =3a(3) =8a(4) =21a(5) =57a(6) =157a(7) =438a(8) =1237a(9) =3530a(10) =10165a(11) =29505a(12) =86243a(13) =253654a(14) =750157a(15) =2229469a(16) =6655369a(17) =19946979a(18) =60000443a(19) =181076982a(20) =548125929a(21) =1663786344a(22) =5063133335a(23) =15444046031a(24) =47211447131a(25) =144614092732a(26) =443803262627