Almost trivalent maps.

Terms

a(0) =4a(1) =32a(2) =200a(3) =1120a(4) =5880a(5) =29568a(6) =144144a(7) =686400a(8) =3208920a(9) =14780480a(10) =67251184a(11) =302865472a(12) =1352078000a(13) =5990745600a(14) =26369978400a(15) =115407434880a(16) =502503206040