Triangular numbers T from A000217 such that (4*T+1)/5 is prime.

Terms

a(0) =6a(1) =21a(2) =36a(3) =66a(4) =91a(5) =136a(6) =171a(7) =351a(8) =496a(9) =561a(10) =741a(11) =946a(12) =1176a(13) =1326a(14) =1596a(15) =2016a(16) =2346a(17) =2701a(18) =2926a(19) =3321a(20) =3486a(21) =4851a(22) =6216a(23) =6441a(24) =7626a(25) =8646a(26) =8911a(27) =9591a(28) =10011a(29) =10296