Diagonal in array of n-gonal numbers A081422.

Terms

a(0) =1a(1) =5a(2) =18a(3) =46a(4) =95a(5) =171a(6) =280a(7) =428a(8) =621a(9) =865a(10) =1166a(11) =1530a(12) =1963a(13) =2471a(14) =3060a(15) =3736a(16) =4505a(17) =5373a(18) =6346a(19) =7430a(20) =8631a(21) =9955a(22) =11408a(23) =12996a(24) =14725a(25) =16601a(26) =18630a(27) =20818a(28) =23171a(29) =25695