Powers of 2 with 3 alternatingly added and subtracted.

Terms

a(0) =4a(1) =-1a(2) =7a(3) =5a(4) =19a(5) =29a(6) =67a(7) =125a(8) =259a(9) =509a(10) =1027a(11) =2045a(12) =4099a(13) =8189a(14) =16387a(15) =32765a(16) =65539a(17) =131069a(18) =262147a(19) =524285a(20) =1048579a(21) =2097149a(22) =4194307a(23) =8388605a(24) =16777219a(25) =33554429a(26) =67108867a(27) =134217725a(28) =268435459a(29) =536870909