Start with 1; multiply alternately by 3 and 4.

Terms

a(0) =1a(1) =3a(2) =12a(3) =36a(4) =144a(5) =432a(6) =1728a(7) =5184a(8) =20736a(9) =62208a(10) =248832a(11) =746496a(12) =2985984a(13) =8957952a(14) =35831808a(15) =107495424a(16) =429981696a(17) =1289945088a(18) =5159780352a(19) =15479341056a(20) =61917364224a(21) =185752092672a(22) =743008370688