a(n) = 7*3^n - 2.

Terms

a(0) =5a(1) =19a(2) =61a(3) =187a(4) =565a(5) =1699a(6) =5101a(7) =15307a(8) =45925a(9) =137779a(10) =413341a(11) =1240027a(12) =3720085a(13) =11160259a(14) =33480781a(15) =100442347a(16) =301327045a(17) =903981139a(18) =2711943421a(19) =8135830267a(20) =24407490805a(21) =73222472419a(22) =219667417261a(23) =659002251787