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For the 2-monad $((-)^2,I,C)$ on CAT, with unit $I$ described by identities and multiplication $C$ described by composition, we show that a functor $F : {\cal K}^2 \rightarrow \cal K$ satisfying $FI_{\cal K} = 1_{\cal K}$ admits a unique, normal, pseudo-algebra structure for $(-)^2$ if and only if there is a mere natural isomorphism $F F^2 \rightarrow F C_{\cal K}$. We show that when this is the case the set of all natural transformations $F F^2 \rightarrow F C_{\cal K}$ forms a commutative monoid isomorphic to the centre of $\cal K$.
@article{TAC_2002_10_a5, author = {Robert Rosebrugh and R.J. Wood}, title = {Coherence for {Factorization} {Algebras}}, journal = {Theory and applications of categories}, pages = {134--147}, publisher = {mathdoc}, volume = {10}, year = {2002}, language = {en}, url = {http://geodesic.mathdoc.fr/item/TAC_2002_10_a5/} }
Robert Rosebrugh; R.J. Wood. Coherence for Factorization Algebras. Theory and applications of categories, Tome 10 (2002), pp. 134-147. http://geodesic.mathdoc.fr/item/TAC_2002_10_a5/