a(n) = s(1) + s(2) + ... + s(n), where s = A026600.

Terms

a(0) =1a(1) =3a(2) =6a(3) =8a(4) =11a(5) =12a(6) =15a(7) =16a(8) =18a(9) =20a(10) =23a(11) =24a(12) =27a(13) =28a(14) =30a(15) =31a(16) =33a(17) =36a(18) =39a(19) =40a(20) =42a(21) =43a(22) =45a(23) =48a(24) =50a(25) =53a(26) =54a(27) =56a(28) =59a(29) =60