a(n)=Product{k=0..n, 1+3^A101650(k)}/2.

Terms

a(0) =1a(1) =2a(2) =8a(3) =16a(4) =160a(5) =640a(6) =6400a(7) =12800a(8) =25600a(9) =256000a(10) =1024000a(11) =4096000a(12) =16384000a(13) =163840000a(14) =327680000a(15) =655360000a(16) =2621440000a(17) =5242880000a(18) =10485760000a(19) =104857600000a(20) =419430400000