Largest prime <= 3^n.

Terms

a(0) =3a(1) =7a(2) =23a(3) =79a(4) =241a(5) =727a(6) =2179a(7) =6553a(8) =19681a(9) =59029a(10) =177131a(11) =531383a(12) =1594301a(13) =4782961a(14) =14348891a(15) =43046623a(16) =129140153a(17) =387420479a(18) =1162261453a(19) =3486784393a(20) =10460353199a(21) =31381059607a(22) =94143178807a(23) =282429536453a(24) =847288609423