a(n) = 4*2^n - 9.

Terms

a(0) =-1a(1) =7a(2) =23a(3) =55a(4) =119a(5) =247a(6) =503a(7) =1015a(8) =2039a(9) =4087a(10) =8183a(11) =16375a(12) =32759a(13) =65527a(14) =131063a(15) =262135a(16) =524279a(17) =1048567a(18) =2097143a(19) =4194295a(20) =8388599a(21) =16777207a(22) =33554423a(23) =67108855a(24) =134217719a(25) =268435447a(26) =536870903a(27) =1073741815a(28) =2147483639a(29) =4294967287