a(n) = 288*2^n - 156.

Terms

a(0) =132a(1) =420a(2) =996a(3) =2148a(4) =4452a(5) =9060a(6) =18276a(7) =36708a(8) =73572a(9) =147300a(10) =294756a(11) =589668a(12) =1179492a(13) =2359140a(14) =4718436a(15) =9437028a(16) =18874212a(17) =37748580a(18) =75497316a(19) =150994788a(20) =301989732a(21) =603979620a(22) =1207959396a(23) =2415918948a(24) =4831838052a(25) =9663676260a(26) =19327352676a(27) =38654705508