Number of friendly 3-watermelons of length n.

Terms

a(0) =1a(1) =0a(2) =2a(3) =6a(4) =24a(5) =110a(6) =550a(7) =2922a(8) =16242a(9) =93520a(10) =553980a(11) =3359384a(12) =20777588a(13) =130696662a(14) =834244830a(15) =5393850898a(16) =35272830054a(17) =233016356788