Number of 4-irreducible maps made up of two hexagons and n squares.

Terms

a(0) =1a(1) =6a(2) =21a(3) =62a(4) =180a(5) =540a(6) =1683a(7) =5418a(8) =17901a(9) =60390a(10) =207207a(11) =720954a(12) =2537964a(13) =9023328a(14) =32354910a(15) =116873028a(16) =424901655a(17) =1553574330a(18) =5709123135a(19) =21075032250a(20) =78114397680a(21) =290595385080