a(n) = a(n-1) + 2a(n-2).

Terms

a(0) =-1a(1) =4a(2) =2a(3) =10a(4) =14a(5) =34a(6) =62a(7) =130a(8) =254a(9) =514a(10) =1022a(11) =2050a(12) =4094a(13) =8194a(14) =16382a(15) =32770a(16) =65534a(17) =131074a(18) =262142a(19) =524290a(20) =1048574a(21) =2097154a(22) =4194302a(23) =8388610a(24) =16777214a(25) =33554434a(26) =67108862a(27) =134217730a(28) =268435454a(29) =536870914