a(n) = n-th centered n-gonal number.

Terms

a(0) =1a(1) =2a(2) =7a(3) =19a(4) =41a(5) =76a(6) =127a(7) =197a(8) =289a(9) =406a(10) =551a(11) =727a(12) =937a(13) =1184a(14) =1471a(15) =1801a(16) =2177a(17) =2602a(18) =3079a(19) =3611a(20) =4201a(21) =4852a(22) =5567a(23) =6349a(24) =7201a(25) =8126a(26) =9127a(27) =10207a(28) =11369a(29) =12616