a(1)=-1, a(2)=2, thereafter a(n) = (1/(2n))*((7n-22)a(n-1)+2(2n-1)a(n-2)).

Terms

a(0) =-1a(1) =2a(2) =-2a(3) =2a(4) =-1a(5) =2a(6) =2a(7) =8a(8) =22a(9) =68a(10) =212a(11) =678a(12) =2207a(13) =7298a(14) =24458a(15) =82928a(16) =284066a(17) =981884a(18) =3421316a(19) =12007556a(20) =42416486a(21) =150718772a(22) =538421588a(23) =1932856592a(24) =6969847484a(25) =25237057112a(26) =91729488352a(27) =334589415278