a(n) = p^2*(p-1), where p = prime(n).

Terms

a(0) =4a(1) =18a(2) =100a(3) =294a(4) =1210a(5) =2028a(6) =4624a(7) =6498a(8) =11638a(9) =23548a(10) =28830a(11) =49284a(12) =67240a(13) =77658a(14) =101614a(15) =146068a(16) =201898a(17) =223260a(18) =296274a(19) =352870a(20) =383688a(21) =486798a(22) =564898a(23) =697048a(24) =903264a(25) =1020100a(26) =1082118a(27) =1213594a(28) =1283148a(29) =1430128