Right leading diagonal of the triangle described in comments.

Terms

a(0) =2a(1) =5a(2) =7a(3) =13a(4) =13a(5) =31a(6) =31a(7) =59a(8) =59a(9) =59a(10) =59a(11) =179a(12) =179a(13) =179a(14) =179a(15) =179a(16) =179a(17) =179a(18) =179a(19) =179a(20) =179a(21) =179a(22) =179a(23) =179a(24) =179a(25) =179a(26) =1381a(27) =1381a(28) =1381a(29) =1381