a(n) = 4*3^n + 1.

Terms

a(0) =5a(1) =13a(2) =37a(3) =109a(4) =325a(5) =973a(6) =2917a(7) =8749a(8) =26245a(9) =78733a(10) =236197a(11) =708589a(12) =2125765a(13) =6377293a(14) =19131877a(15) =57395629a(16) =172186885a(17) =516560653a(18) =1549681957a(19) =4649045869a(20) =13947137605a(21) =41841412813a(22) =125524238437a(23) =376572715309