a(n) = 3*(-1)^(n+1)*2^n - 1.

Terms

a(0) =-4a(1) =5a(2) =-13a(3) =23a(4) =-49a(5) =95a(6) =-193a(7) =383a(8) =-769a(9) =1535a(10) =-3073a(11) =6143a(12) =-12289a(13) =24575a(14) =-49153a(15) =98303a(16) =-196609a(17) =393215a(18) =-786433a(19) =1572863a(20) =-3145729a(21) =6291455a(22) =-12582913a(23) =25165823a(24) =-50331649a(25) =100663295a(26) =-201326593a(27) =402653183a(28) =-805306369a(29) =1610612735