1,3

By 'binary tree' we mean a rooted, ordered tree in which each vertex has either 0 or 2 children.

a(n) is also the number of Dyck words of semilength n-1 with no DDUUU.

Eric S. Rowland, Pattern avoidance in binary trees.

Generating function f(x) satisfies (x-2) x f(x)^2 + (2 x^2 - 2 x + 1) f(x) + (x-1) x = 0

Sequence in context: A161898 A159770 A159773 this_sequence A159771 A159768 A128739

Adjacent sequences: A159766 A159767 A159768 this_sequence A159770 A159771 A159772

nonn

Eric S Rowland (erowland(AT)math.rutgers.edu), Apr 23 2009