a(n) = 140*2^n - 112.

Terms

a(0) =28a(1) =168a(2) =448a(3) =1008a(4) =2128a(5) =4368a(6) =8848a(7) =17808a(8) =35728a(9) =71568a(10) =143248a(11) =286608a(12) =573328a(13) =1146768a(14) =2293648a(15) =4587408a(16) =9174928a(17) =18349968a(18) =36700048a(19) =73400208a(20) =146800528a(21) =293601168a(22) =587202448a(23) =1174405008a(24) =2348810128a(25) =4697620368a(26) =9395240848a(27) =18790481808a(28) =37580963728a(29) =75161927568