Expansion of g.f.: sqrt(1+6*x+x^2).

Terms

a(0) =1a(1) =3a(2) =-4a(3) =12a(4) =-44a(5) =180a(6) =-788a(7) =3612a(8) =-17116a(9) =83172a(10) =-412196a(11) =2075436a(12) =-10586892a(13) =54595476a(14) =-284157492a(15) =1490774076a(16) =-7875206076a(17) =41854313412a(18) =-223636052036