a(n) = 2^n - 4.

Terms

a(0) =0a(1) =4a(2) =12a(3) =28a(4) =60a(5) =124a(6) =252a(7) =508a(8) =1020a(9) =2044a(10) =4092a(11) =8188a(12) =16380a(13) =32764a(14) =65532a(15) =131068a(16) =262140a(17) =524284a(18) =1048572a(19) =2097148a(20) =4194300a(21) =8388604a(22) =16777212a(23) =33554428a(24) =67108860a(25) =134217724a(26) =268435452a(27) =536870908a(28) =1073741820a(29) =2147483644