I am asking because the literature seems to contain some inconsistencies as to the definition of a braided monoidal category, and I'd like to get it straight. According Chari and Pressley's book ``A guide to quantum groups," a braided monoidal category is a monoidal category $\mathcal{C}$ along with a natural system of isomorphisms $\sigma\_{U,V}: U \otimes V \rightarrow V \otimes U$ for all pairs of objects $U$ and $V$, such that


(i) The ``Hexagon" axioms (two commutative diagrams) hold. 


(ii) The ``identity object" axioms: $\rho\_V= \lambda\_V \circ \sigma\_{{\bf 1},V}: {\bf 1} \otimes V \rightarrow V$
and
$\lambda\_V= \rho\_V \circ \sigma\_{V, {\bf 1}}: {V} \otimes {\bf 1} \rightarrow V$,
where $\lambda\_V$ and $\rho\_V$ are the isomorphisms of $V \otimes {\bf 1}$ and ${\bf 1} \otimes V$ with $V$ that are part of the definition of monoidal category. See Chari-Pressley Definitions 5.2.1 and 5.2.4. They use the term "quasitensor category," but note on p153 that the term "braided monoidal category" is equivalent.


However, in some references (ii) seems to have been dropped. I am thinking in particular of Definition 3.1 is this
[expository paper](http://arxiv.org/abs/0804.4688), 
and the
[wikipedia article](http://en.wikipedia.org/w/index.php?title=Braided_monoidal_category&oldid=309075821). 
The wikipedia article goes further, and suggests that (ii) somehow follows from (i) and the axioms of a monoidal category. So, my questions are.


1) Is (ii) needed? That is if we do not impose (ii), does it follow from (i) and the axioms of a monoidal category?


2) If (ii) is needed, can someone provide an example demonstrating why? That is, provide an example of a monoidal category $\mathcal{C}$ along with maps $\sigma\_{U,V}$ such that (i) holds but (ii) fails. Alternatively, if (ii) is not needed, I'd like a proof (or reference to a proof) that it follows from other axioms.



Are the “identity object axioms” in the definition of a braided monoidal category needed? (Answered: No)
This is Proposition 1 in the seminal paper "Braided Monoidal Categories" by Joyal and Street. Relation (ii) is implied by the others.


