a(n)=-a(n-1)+3*a(n-2), n>1 ; a(0)=1, a(1)=-3 .

Terms

a(0) =1a(1) =-3a(2) =6a(3) =-15a(4) =33a(5) =-78a(6) =177a(7) =-411a(8) =942a(9) =-2175a(10) =5001a(11) =-11526a(12) =26529a(13) =-61107a(14) =140694a(15) =-324015a(16) =746097a(17) =-1718142a(18) =3956433a(19) =-9110859a(20) =20980158a(21) =-48312735a(22) =111253209a(23) =-256191414a(24) =589951041a(25) =-1358525283a(26) =3128378406a(27) =-7203954255a(28) =16589089473