Diagonal in array of n-gonal numbers A081422.

Terms

a(0) =1a(1) =10a(2) =33a(3) =76a(4) =145a(5) =246a(6) =385a(7) =568a(8) =801a(9) =1090a(10) =1441a(11) =1860a(12) =2353a(13) =2926a(14) =3585a(15) =4336a(16) =5185a(17) =6138a(18) =7201a(19) =8380a(20) =9681a(21) =11110a(22) =12673a(23) =14376a(24) =16225a(25) =18226a(26) =20385a(27) =22708a(28) =25201a(29) =27870