a(n) = 6*a(n-1) - 6*a(n-2), with a(0)=1, a(1)=3.

Terms

a(0) =1a(1) =3a(2) =12a(3) =54a(4) =252a(5) =1188a(6) =5616a(7) =26568a(8) =125712a(9) =594864a(10) =2814912a(11) =13320288a(12) =63032256a(13) =298271808a(14) =1411437312a(15) =6678993024a(16) =31605334272a(17) =149558047488a(18) =707716279296