Diagonal in array of n-gonal numbers A081422.

Terms

a(0) =1a(1) =11a(2) =36a(3) =82a(4) =155a(5) =261a(6) =406a(7) =596a(8) =837a(9) =1135a(10) =1496a(11) =1926a(12) =2431a(13) =3017a(14) =3690a(15) =4456a(16) =5321a(17) =6291a(18) =7372a(19) =8570a(20) =9891a(21) =11341a(22) =12926a(23) =14652a(24) =16525a(25) =18551a(26) =20736a(27) =23086a(28) =25607a(29) =28305