a(n) = index of the triangular number A076971(n).

Terms

a(0) =1a(1) =2a(2) =3a(3) =5a(4) =7a(5) =10a(6) =15a(7) =21a(8) =30a(9) =43a(10) =61a(11) =87a(12) =123a(13) =174a(14) =247a(15) =349a(16) =494a(17) =699a(18) =989a(19) =1399a(20) =1979a(21) =2799a(22) =3959a(23) =5599a(24) =7919a(25) =11199a(26) =15838a(27) =22399a(28) =31677a(29) =44799