a(n) = index of triangular number A077002(n).

Terms

a(0) =1a(1) =1a(2) =1a(3) =2a(4) =4a(5) =9a(6) =22a(7) =59a(8) =167a(9) =501a(10) =1585a(11) =5258a(12) =18215a(13) =65676a(14) =245738a(15) =951740a(16) =3806961a(17) =15696503a(18) =66594623a(19) =290279233a(20) =1298168196a(21) =5948954024a(22) =27903067710a(23) =133818411722a(24) =655573653819