Start with 1; multiply alternately by 4 and 3.

Terms

a(0) =1a(1) =4a(2) =12a(3) =48a(4) =144a(5) =576a(6) =1728a(7) =6912a(8) =20736a(9) =82944a(10) =248832a(11) =995328a(12) =2985984a(13) =11943936a(14) =35831808a(15) =143327232a(16) =429981696a(17) =1719926784a(18) =5159780352a(19) =20639121408a(20) =61917364224a(21) =247669456896a(22) =743008370688