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 $\cal K$.
Keywords: coherence, factorization algebra.
2000 MSC: 18A32, 18D05.
Theory and Applications of Categories, Vol. 10, 2002, No. 6, pp 134-147.
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