4 times heptagonal numbers: a(n) = 2*n*(5*n-3).

Terms

a(0) =0a(1) =4a(2) =28a(3) =72a(4) =136a(5) =220a(6) =324a(7) =448a(8) =592a(9) =756a(10) =940a(11) =1144a(12) =1368a(13) =1612a(14) =1876a(15) =2160a(16) =2464a(17) =2788a(18) =3132a(19) =3496a(20) =3880a(21) =4284a(22) =4708a(23) =5152a(24) =5616a(25) =6100a(26) =6604a(27) =7128a(28) =7672a(29) =8236