Numbers of tree alignments.

Terms

a(0) =0a(1) =0a(2) =2a(3) =6a(4) =22a(5) =88a(6) =370a(7) =1612a(8) =7232a(9) =33304a(10) =157102a(11) =757804a(12) =3731352a(13) =18720504a(14) =95519428a(15) =494733144a(16) =2596388976a(17) =13783481424a(18) =73906300822a(19) =399722732236