# Parsing problem 1.D.5 in MWG

I am a little confused about the statement of 1.D.5 in MWG, which I will reproduce here for convenience. I have "solved" the problem, I just don't understand something particular.

$$\textbf{(1.D.5)}$$ Let $$X = \{x,y,z\}$$ and $$\mathscr{B} = \{ \{x,y\},\{y,z\},\{z,x\} \}$$. Suppose that choice is now stochastic in the sense that, for every $$B \in \mathscr{B}$$, $$C(B)$$ is a frequency distribution over alternatives in $$B$$. For example, if $$B = \{x,y\}$$, we write $$C(B) = (C_x(B), C_y(B))...$$

So, my issue is that it seems that $$C : \mathscr{B} \to \mathbb{R}_+^2$$ is not well-defined, and would be more appropriately defined as a function $$A (\subseteq X^2) \to \mathbb{R}_+^2$$. What am I missing?

• On second thought, I suppose any function is well-defined if you explicitly enumerate each input/output pair. But the problem doesn’t do that. – Seh-kai Sep 5 '20 at 11:29
• Isn't $\mathscr B\subset X^2$? The book also specifies the range of the $C$ function as the set of non-negative 2-vectors whose elements sum to $1$. – Herr K. Sep 5 '20 at 17:17
• I believe $\mathscr{B}$ is a subset of the powerset of $X$; i.e. all subsets of $X$. My issue is that I don't know what $C(\{ z,x \})$ is supposed to be. Is it $= (C_z(B),C_x(B))?$ Or is it $= (C_x(B),C_z(B))$? The outputs of $C$ are ordered based on the elements of $B$, but since $B$ is a set, $B$ carries no ordering. – Seh-kai Sep 5 '20 at 19:29
• You're right about $B$ having no order. I missed that part. – Herr K. Sep 5 '20 at 20:55