Show that if $X_1$ and $X_2$ are preferentially independent of $X_3$, and $X_2$ and $X_3$ are preferentially independent of $X_1$, then $X_3$ and $X_1$ are preferentially independent of $X_2$.

Alex is given the choice between two games. In Game 1, a fair coin is
flipped and if it comes up heads, Alex receives $\$100$. If the coin comes
up tails, Alex receives nothing. In Game 2, a fair coin is flipped
twice. Each time the coin comes up heads, Alex receives $\$50$, and Alex
receives nothing for each coin flip that comes up tails. Assuming that
Alex has a monotonically increasing utility function for money in the
range \[\$0, \$100\], show mathematically that if Alex prefers Game 2 to
Game 1, then Alex is risk averse (at least with respect to this range of
monetary amounts).

Show that if $X_1$ and $X_2$ are preferentially independent of $X_3$,
and $X_2$ and $X_3$ are preferentially independent of $X_1$, then $X_3$
and $X_1$ are preferentially independent of $X_2$.