A Calkin-Wilf tree is a special type of binary tree obtained by starting with the fraction and iteratively adding and below each fraction . The Stern-Brocot tree is closely related, putting and below each fraction . Both trees generate every rational number. Writing out the terms in sequence gives 1/1, 1/2, 2/1, 1/3, 3/2, 2/3, 3/1, 1/4, 4/3, 3/5, 5/2, 2/5, 5/3, 3/4, 4/1, ...The sequence has the property that each denominator is the next numerator. This sequence, 1, 1, 2, 1, 3, 2, 3, 1, 4, 3, 5, 2, 5, 3, 4, ... (Sloane's A002487), is known as Stern's diatomic series, or the fusc function (Dijkstra 1982).