Using Catalan words and a q-shuffle algebra to describe a PBW basis for the positive part of U_q({widehat {mathfrak{sl}}}₂)

The positive part $U^+_q$ of $U_q({\widehat {\mathfrak{sl}}}_2)$ has a presentation with two generators $A,B$ that satisfy the cubic $q$-Serre relations. In 1993 I. Damiani obtained a PBW basis for $U^+_q$, consisting of some elements $\lbrace E_{n\delta+\alpha_0}\rbrace_{n=0}^\infty$, $\lbrace E_{n\delta+\alpha_1}\rbrace_{n=0}^\infty$, $\lbrace E_{n\delta}\rbrace_{n=1}^\infty$ that are defined recursively. Our goal is to describe these elements in closed form. We achieve this goal using Catalan words and a $q$-shuffle algebra.