a(n) = p*(p - 1), where p is A000043(n).

Terms

a(0) =2a(1) =6a(2) =20a(3) =42a(4) =156a(5) =272a(6) =342a(7) =930a(8) =3660a(9) =7832a(10) =11342a(11) =16002a(12) =270920a(13) =367842a(14) =1634562a(15) =4851006a(16) =5200680a(17) =10345872a(18) =18083756a(19) =19558506a(20) =93867032a(21) =98813540a(22) =125720156a(23) =397464032a(24) =470911700a(25) =538634472a(26) =1979938512a(27) =7437768806a(28) =12210802506