a(n) = P(n)^3 - P(n)^2 where P(n) = A000931(n).

Terms

a(0) =0a(1) =0a(2) =0a(3) =0a(4) =0a(5) =0a(6) =0a(7) =0a(8) =4a(9) =4a(10) =18a(11) =48a(12) =100a(13) =294a(14) =648a(15) =1584a(16) =3840a(17) =8820a(18) =21168a(19) =49284a(20) =115248a(21) =270400a(22) =628660a(23) =1468548a(24) =3420150a(25) =7960000a(26) =18539400a(27) =43120350a(28) =100328400a(29) =233365440