a(n) = 492*2^n - 222.

Terms

a(0) =270a(1) =762a(2) =1746a(3) =3714a(4) =7650a(5) =15522a(6) =31266a(7) =62754a(8) =125730a(9) =251682a(10) =503586a(11) =1007394a(12) =2015010a(13) =4030242a(14) =8060706a(15) =16121634a(16) =32243490a(17) =64487202a(18) =128974626a(19) =257949474a(20) =515899170a(21) =1031798562a(22) =2063597346a(23) =4127194914a(24) =8254390050a(25) =16508780322a(26) =33017560866a(27) =66035121954a(28) =132070244130