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\newcommand{\courseName}{Math 480A2}
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\newcommand{\hwDueDate}{October 13, 2022}


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\begin{document}

\begin{center}
	{\large \courseName, Homework \hwNumber}\\
	Due \hwDueDate\\
\end{center}

{\small \textit{Homework is graded out of a total of 10 points.  Collaboration is permitted, but you must list all coauthors on a problem's solution at the top of the page, and your writing must be your own.}}

\vspace{5mm}

\textbf{Problem 1.} (6 points) In the following, consider the random outcomes of a fair 12-sided die, where ``fair'' here means that each outcome has an equal chance of occurring.
\begin{enumerate}
	\item Describe a discrete probability space which represents the situation given above, including a set $X$ of outcomes, and a probability function describing the likelihood of each outcome.

	\item Let $D$ be the random variable on $X$ mapping each outcome to its number of positive divisors.  (E.g.\ $R(6) = 4$ since 6 is divisible by 1, 2, 3, and 6.)  Compute the expected value of $D$.
	\setcounter{saveenum}{\value{enumi}}
\end{enumerate}

The random variable $D$ allows us to define a probability function on the set of its possible values as follows: if $d$ is a positive integer representing a possible number of divisors, then define $\Prob(d) = \Prob[D = d]$, where the right side of this equation is the probability of the random variable $D$ taking value $d$ over the original probability space $X$.  This probability function is called the \emph{push-forward} distribution of $X$ by the random variable $D$.

\begin{enumerate}
	\setcounter{enumi}{\value{saveenum}}
	\item Compute the push-forward distribution of $X$ by the random variable $D$, expressed as a probability function $p$ on the set $Y = \{1, 2, 3, \ldots\}$ of positive integers.  (\textit{Hint}.  In other words, compute the values of the function $p(d) = \Prob[D = d]$ for each positive integer $d$.)
\end{enumerate}

\vspace{5mm}

\textbf{Problem 2.} (4 points) Now consider the following random system: we repeatedly roll a fair 4-sided die until a 4 is rolled, at which point we stop, and we record the sequence of rolls as our outcome.  Thus if we rolled a 3 and then a 4, the outcome would be the finite sequence $(3, 4)$, and if we rolled a 2, followed by a 1, followed by a 2, followed by a 4, then the outcome would be the sequence $(2, 1, 2, 4)$.
\begin{enumerate}
	\item Describe a discrete probability space which represents the situation given above, including a set $X$ of outcomes, and a probability function $p$ describing the likelihood of each outcome.  Show that $\sum_{x \in X} p(x) = 1$ for this set $X$ and probability function $p$.

	\item Let $L$ be the random variable on $X$ mapping each outcome to its number of rolls, i.e.\ to its length as a sequence.  (E.g.\ $L((2, 1, 2, 4)) = 4$.)  Using Markov's inequality and the fact that the expected value of $L$ is 4, find an upper bound on the probability that a sequence of rolls has length at least 10.
\end{enumerate}

\textbf{Challenge.} (1 bonus point) Prove that the random variable $L$ from the previous problem has expected value 4.

\end{document}