a(n) = 21*2^n - 15.

Terms

a(0) =6a(1) =27a(2) =69a(3) =153a(4) =321a(5) =657a(6) =1329a(7) =2673a(8) =5361a(9) =10737a(10) =21489a(11) =42993a(12) =86001a(13) =172017a(14) =344049a(15) =688113a(16) =1376241a(17) =2752497a(18) =5505009a(19) =11010033a(20) =22020081a(21) =44040177a(22) =88080369a(23) =176160753a(24) =352321521a(25) =704643057a(26) =1409286129a(27) =2818572273a(28) =5637144561a(29) =11274289137