Expansion of g.f.: (1-x)*(1+2*x)/((1+x)*(1-3*x+x^2)).

Terms

a(0) =1a(1) =3a(2) =6a(3) =17a(4) =43a(5) =114a(6) =297a(7) =779a(8) =2038a(9) =5337a(10) =13971a(11) =36578a(12) =95761a(13) =250707a(14) =656358a(15) =1718369a(16) =4498747a(17) =11777874a(18) =30834873a(19) =80726747a(20) =211345366a(21) =553309353a(22) =1448582691a(23) =3792438722a(24) =9928733473a(25) =25993761699a(26) =68052551622a(27) =178163893169