a(n) = 2^(1 + floor(n*log_2(3))) - (3^n + 1).

Terms

a(0) =0a(1) =0a(2) =6a(3) =4a(4) =46a(5) =12a(6) =294a(7) =1908a(8) =1630a(9) =13084a(10) =6486a(11) =84996a(12) =517134a(13) =502828a(14) =3605638a(15) =2428308a(16) =24062142a(17) =5077564a(18) =149450422a(19) =985222180a(20) =808182894a(21) =6719515980a(22) =2978678758a(23) =43295774644a(24) =267326277406a(25) =252223018332