a(0)=3; thereafter a(n) = 20*6^(n-1)-2^(n-1).

Terms

a(0) =3a(1) =19a(2) =118a(3) =716a(4) =4312a(5) =25904a(6) =155488a(7) =933056a(8) =5598592a(9) =33592064a(10) =201553408a(11) =1209322496a(12) =7255939072a(13) =43535642624a(14) =261213872128