A categorification of the Beilinson-Lusztig-MacPherson form of the quantum
sl(2) was constructed in the paper arXiv:0803.3652 by the second author. Here
we enhance the graphical calculus introduced and developed in that paper to
include two-morphisms between divided powers one-morphisms and their
compositions. We obtain explicit diagrammatical formulas for the decomposition
of products of divided powers one-morphisms as direct sums of indecomposable
one-morphisms; the latter are in a bijection with the Lusztig canonical basis
elements. These formulas have integral coefficients and imply that one of the
main results of Lauda's paper---identification of the Grothendieck ring of his
2-category with the idempotented quantum sl(2)---also holds when the 2-category
is defined over the ring of integers rather than over a field.Comment: 72 pages, LaTeX2e with xypic and pstricks macro
