a(n) = position of the n-th n in A026400.

Terms

a(0) =1a(1) =4a(2) =9a(3) =17a(4) =27a(5) =38a(6) =53a(7) =71a(8) =88a(9) =111a(10) =134a(11) =161a(12) =186a(13) =219a(14) =255a(15) =289a(16) =322a(17) =365a(18) =406a(19) =454a(20) =496a(21) =549a(22) =605a(23) =652a(24) =710a(25) =767a(26) =830a(27) =886a(28) =945a(29) =1027