a(n) = 4 - 3*2^n.

Terms

a(0) =1a(1) =-2a(2) =-8a(3) =-20a(4) =-44a(5) =-92a(6) =-188a(7) =-380a(8) =-764a(9) =-1532a(10) =-3068a(11) =-6140a(12) =-12284a(13) =-24572a(14) =-49148a(15) =-98300a(16) =-196604a(17) =-393212a(18) =-786428a(19) =-1572860a(20) =-3145724a(21) =-6291452a(22) =-12582908a(23) =-25165820a(24) =-50331644a(25) =-100663292a(26) =-201326588a(27) =-402653180a(28) =-805306364a(29) =-1610612732