a(n) = 21*2^n.

Terms

a(0) =21a(1) =42a(2) =84a(3) =168a(4) =336a(5) =672a(6) =1344a(7) =2688a(8) =5376a(9) =10752a(10) =21504a(11) =43008a(12) =86016a(13) =172032a(14) =344064a(15) =688128a(16) =1376256a(17) =2752512a(18) =5505024a(19) =11010048a(20) =22020096a(21) =44040192a(22) =88080384a(23) =176160768a(24) =352321536a(25) =704643072a(26) =1409286144a(27) =2818572288a(28) =5637144576a(29) =11274289152