The constant coefficient of (x + x*y + y + 1/(x*y))^n.

Terms

a(0) =1a(1) =0a(2) =2a(3) =6a(4) =6a(5) =60a(6) =110a(7) =420a(8) =1750a(9) =4200a(10) =19152a(11) =60060a(12) =201894a(13) =792792a(14) =2525952a(15) =9525516a(16) =33886710a(17) =117738192a(18) =439904036a(19) =1544744916a(20) =5628776296a(21) =20535629400a(22) =73621352532a(23) =270821996016a(24) =982153129126