a(n) = position of 3*(n^2) in A000408.

Terms

a(0) =1a(1) =5a(2) =14a(3) =28a(4) =48a(5) =73a(6) =103a(7) =138a(8) =179a(9) =226a(10) =277a(11) =333a(12) =395a(13) =461a(14) =531a(15) =608a(16) =691a(17) =777a(18) =869a(19) =966a(20) =1068a(21) =1175a(22) =1287a(23) =1403a(24) =1526a(25) =1653a(26) =1783a(27) =1921a(28) =2063a(29) =2209