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

Terms

a(0) =-3a(1) =1a(2) =-9a(3) =25a(4) =-33a(5) =49a(6) =-129a(7) =289a(8) =-513a(9) =961a(10) =-2049a(11) =4225a(12) =-8193a(13) =16129a(14) =-32769a(15) =66049a(16) =-131073a(17) =261121a(18) =-524289a(19) =1050625a(20) =-2097153a(21) =4190209a(22) =-8388609a(23) =16785409a(24) =-33554433a(25) =67092481a(26) =-134217729a(27) =268468225a(28) =-536870913a(29) =1073676289