a(n) = n_t(n) where t() = triangular numbers A000217.

Terms

a(0) =1a(1) =2a(2) =3a(3) =4a(4) =5a(5) =6a(6) =7a(7) =8a(8) =9a(9) =55a(10) =67a(11) =80a(12) =94a(13) =109a(14) =125a(15) =142a(16) =160a(17) =179a(18) =199a(19) =420a(20) =463a(21) =508a(22) =555a(23) =604a(24) =655a(25) =708a(26) =763a(27) =820a(28) =879a(29) =1395