Constant term in the expansion of (1 + x + y + z + 1/(x*y*z))^n.

Terms

a(0) =1a(1) =1a(2) =1a(3) =1a(4) =25a(5) =121a(6) =361a(7) =841a(8) =4201a(9) =25705a(10) =118441a(11) =423721a(12) =1628881a(13) =8065201a(14) =41225185a(15) =184416961a(16) =768211081a(17) =3420474121a(18) =16620237001a(19) =79922011465a(20) =364149052705