a(n) = (3^n+1)/(3-(-1)^n).

Terms

a(0) =1a(1) =1a(2) =5a(3) =7a(4) =41a(5) =61a(6) =365a(7) =547a(8) =3281a(9) =4921a(10) =29525a(11) =44287a(12) =265721a(13) =398581a(14) =2391485a(15) =3587227a(16) =21523361a(17) =32285041a(18) =193710245a(19) =290565367a(20) =1743392201a(21) =2615088301a(22) =15690529805a(23) =23535794707a(24) =141214768241a(25) =211822152361