G.f.: (-1+6*x)/(1-3*x-2*x^2).

Terms

a(0) =-1a(1) =3a(2) =7a(3) =27a(4) =95a(5) =339a(6) =1207a(7) =4299a(8) =15311a(9) =54531a(10) =194215a(11) =691707a(12) =2463551a(13) =8774067a(14) =31249303a(15) =111296043a(16) =396386735a(17) =1411752291a(18) =5028030343a(19) =17907595611a(20) =63778847519a(21) =227151733779a(22) =809012896375