Skip to main content
1 of 2
Gerald Edgar
  • 41.1k
  • 5
  • 125
  • 219

While Gabe's answer is deleted ("One shouldn't try to work in ZF at 5am"), let me work in ZFC.

(a) The usual middle-thirds Cantor set $C$ is nowhere dense in $\mathbb R$. It has cardinal $\mathfrak c = 2^{\aleph_0}$. So every subset of $C$ is again nowhere dense in $\mathbb R$. Every subset of $C$ has the property of Baire. There are at least $2^{\mathfrak c}$ sets with the property of Baire. [In fact, there are at least $2^{\mathfrak c}$ sets with the property of Baire.]

(b) Start with any family $\mathscr U$ of sets, and define $$ \mathcal A(\mathscr U) = \{ \mathcal A(\mathcal X) : \mathcal{X} = \langle X_s : s \in {}^{<\omega}\omega\rangle , X_s \in \mathscr U \text{ for all } s \in {}^{<\omega}\omega\} . $$ The Suslin operation is idempotent. That is, if $\mathscr U$ is any family of sets, then $\mathcal A(\mathcal A(\mathscr U)) = \mathcal A(\mathscr(U)$.
The family of "Suslin measurable sets" (a.k.a. coanalytic sets) is $\mathcal A(\mathscr G)$, where $\mathcal G$ is the family of all open subsets of $\mathbb R$.

(c) Let $\mathscr G_0 = \{(a,b) : a,b\in\mathbb Q, a<b\}$. Then $\mathcal A(\mathscr G) = \mathcal A(\mathscr G_0)$. Since $\mathscr G_0$ is countable, we conclude that $\mathcal A(\mathscr G_0)$ has cardinal at most $\mathfrak c$. So there are at most $\mathfrak c$ Suslin measurable sets in $\mathbb R$. [In fact, there are exactly $\mathfrak c$ Suslin measurable sets.]

(d) Conclude there is a set with the property of Baire that is not Suslin measurable.

Gerald Edgar
  • 41.1k
  • 5
  • 125
  • 219