a(n) = 3 - 2*3^n.

Terms

a(0) =1a(1) =-3a(2) =-15a(3) =-51a(4) =-159a(5) =-483a(6) =-1455a(7) =-4371a(8) =-13119a(9) =-39363a(10) =-118095a(11) =-354291a(12) =-1062879a(13) =-3188643a(14) =-9565935a(15) =-28697811a(16) =-86093439a(17) =-258280323a(18) =-774840975a(19) =-2324522931a(20) =-6973568799a(21) =-20920706403a(22) =-62762119215a(23) =-188286357651