a(n) = n*(7*n-3)/2.

Terms

a(0) =0a(1) =2a(2) =11a(3) =27a(4) =50a(5) =80a(6) =117a(7) =161a(8) =212a(9) =270a(10) =335a(11) =407a(12) =486a(13) =572a(14) =665a(15) =765a(16) =872a(17) =986a(18) =1107a(19) =1235a(20) =1370a(21) =1512a(22) =1661a(23) =1817a(24) =1980a(25) =2150a(26) =2327a(27) =2511a(28) =2702a(29) =2900