The positive part Uq+ of Uq(slˆ2) has a presentation with two generators A,B that satisfy the cubic q-Serre relations. In 1993 I. Damiani obtained a PBW basis for Uq+, consisting of some elements {Enδ+α0}n=0∞, {Enδ+α1}n=0∞, {Enδ}n=1∞ that are defined recursively. Our goal is to describe these elements in closed form. To reach our goal, start with the free associative algebra V on two generators x,y. The standard (linear) basis for V consists of the words in x,y. In 1995 M. Rosso introduced an associative algebra structure on V, called a q-shuffle algebra. For u,v∈{x,y} their q-shuffle product is u⋆v=uv+q〈u,v〉vu, where 〈u,v〉=2 (resp. 〈u,v〉=−2) if u=v (resp. u≠v). Rosso gave an injective algebra homomorphism ♮ from Uq+ into the q-shuffle algebra V, that sends A↦x and B↦y. We apply ♮ to the above PBW basis, and express the image in the standard basis for V. This image involves words of the following type. Define x‾=1 and y‾=−1. A word a1a2⋯an is Catalan whenever a‾1+a‾2+⋯+a‾i is nonnegative for 1≤i≤n−1 and zero for i=n. In this case n is even. For n≥0 defineCn=∑a1a2⋯a2n[1]q[1+a‾1]q×[1+a‾1+a‾2]q⋯[1+a‾1+a‾2+⋯+a‾2n]q,where the sum is over all the Catalan words a1a2⋯a2n in V that have length 2n. We show that ♮ sends Enδ+α0↦q−2n(q−q−1)2nxCn and Enδ+α1↦q−2n(q−q−1)2nCny for n≥0, and Enδ↦−q−2n(q−q−1)2n−1Cn for n≥1. It follows from this and earlier results of Damiani that {Cn}n=1∞ mutually commute in the q-shuffle algebra V.
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