(2) Give an example of an algebra A in P(N) which is not a $\sigma$-algebra, and an example of a function $\rho$: A $\to$ [0, 1], such that $\rho(\emptyset)$ = 0, and $\rho(N)$ = 1, for which the outer measure $\rho^*$ is identically equal to zero.
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This is not a $\sigma$-algebra because it does not contain the countable union of disjoint sets. For example, consider the sets $A_i = \{i\}$ for $i \in N$. Each $A_i$ is in A, but $\bigcup_{i=1}^\infty A_i = N$, which is in A, but the union of infinitely many Show more…
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