a(n) = 2*(-1)^n - (-5)^n.

Terms

a(0) =1a(1) =3a(2) =-23a(3) =123a(4) =-623a(5) =3123a(6) =-15623a(7) =78123a(8) =-390623a(9) =1953123a(10) =-9765623a(11) =48828123a(12) =-244140623a(13) =1220703123a(14) =-6103515623a(15) =30517578123a(16) =-152587890623a(17) =762939453123