G.f.: x(2-5x-2x^2)/(1-6x+9x^2-x^4).

Terms

a(0) =0a(1) =2a(2) =7a(3) =22a(4) =69a(5) =218a(6) =694a(7) =2224a(8) =7167a(9) =23204a(10) =75415a(11) =245878a(12) =803700a(13) =2632502a(14) =8637127a(15) =28376122a(16) =93326289a(17) =307205138a(18) =1011931354a(19) =3335118004a(20) =10996652127a(21) =36271055864a(22) =119668397395a(23) =394905999598