a(n) = prime(n*(n+1)/2).

Terms

a(0) =2a(1) =5a(2) =13a(3) =29a(4) =47a(5) =73a(6) =107a(7) =151a(8) =197a(9) =257a(10) =317a(11) =397a(12) =467a(13) =571a(14) =659a(15) =769a(16) =883a(17) =1019a(18) =1151a(19) =1291a(20) =1453a(21) =1607a(22) =1783a(23) =1987a(24) =2153a(25) =2371a(26) =2593a(27) =2791a(28) =3037a(29) =3307