a(n) = 2*a(n-1) + a(n-2) - 2*a(n-3) for n>2, a(0)=-1, a(1)=-2, a(2)=-4.

Terms

a(0) =-1a(1) =-2a(2) =-4a(3) =1a(4) =2a(5) =13a(6) =26a(7) =61a(8) =122a(9) =253a(10) =506a(11) =1021a(12) =2042a(13) =4093a(14) =8186a(15) =16381a(16) =32762a(17) =65533a(18) =131066a(19) =262141a(20) =524282a(21) =1048573a(22) =2097146a(23) =4194301a(24) =8388602a(25) =16777213a(26) =33554426a(27) =67108861a(28) =134217722a(29) =268435453