### Abstract

This entry formalizes the coproduct measure.
Let $I$ be a set and $\{M_i\}_{i\in I}$ measurable spaces.
The $\sigma$-algebra
on $\coprod_{i\in I} M_i = \{(i,x)\mid i\in I\land x\in M_i\}$
is defined as the least one making $(\lambda x.\: (i,x))$ measurable for all $i\in I$.
Let $\mu_i$ be measures on $M_i$ for all $i\in I$ and $A$ a measurable set of $\coprod_{i\in I} M_i$.
The coproduct measure $\coprod_{i\in I} \mu_i$ is defined as follows:
\[\left(\coprod_{i\in I} \mu_i\right)(A) = \sum_{i\in I} \mu_i(A_i), \quad \text{where $A_i = \{x\mid (i,x)\in A\}$.}\]
We also prove the relationship with coproduct quasi-Borel spaces:
the functor $R: \mathbf{Meas}\to\mathbf{QBS}$ preserves countable coproducts.