Diagonal of triangular spiral in A051682.

Terms

a(0) =3a(1) =16a(2) =38a(3) =69a(4) =109a(5) =158a(6) =216a(7) =283a(8) =359a(9) =444a(10) =538a(11) =641a(12) =753a(13) =874a(14) =1004a(15) =1143a(16) =1291a(17) =1448a(18) =1614a(19) =1789a(20) =1973a(21) =2166a(22) =2368a(23) =2579a(24) =2799a(25) =3028a(26) =3266a(27) =3513a(28) =3769a(29) =4034