Diagonal of triangular spiral in A051682.

Terms

a(0) =1a(1) =9a(2) =26a(3) =52a(4) =87a(5) =131a(6) =184a(7) =246a(8) =317a(9) =397a(10) =486a(11) =584a(12) =691a(13) =807a(14) =932a(15) =1066a(16) =1209a(17) =1361a(18) =1522a(19) =1692a(20) =1871a(21) =2059a(22) =2256a(23) =2462a(24) =2677a(25) =2901a(26) =3134a(27) =3376a(28) =3627a(29) =3887