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\begin{center}
\LARGE
\textbf{CS 473: Algorithms, Fall 2016}
\\[0.5ex]
\textbf{HW 10 (due Wednesday, November 30 at 8pm)}
\end{center}

\noindent
This homework contains three problems. {\bf Read the instructions for
  submitting homework on the course webpage}. 

\noindent {\bf Collaboration Policy:} For this home work, each student
can work in a group with up to three members. Only one solution for
each group needs to be submitted. Follow the submission instructions
carefully.

\hrule
\medskip\noindent
For problems that ask to prove that a given problem $X$ is NP-hard, a full-credit solution requires the following components:
\begin{itemize}\itemsep0pt
\item
Specify a known {NP}-hard problem $Y$, taken from the problems listed in the notes.
\item
Describe a polynomial-time algorithm for $Y$, using a black-box polynomial-time algorithm for $X$ as a subroutine.  Most {NP}-hardness reductions have the following form: Given an arbitrary instance of $Y$, describe how to transform it into an instance of $X$, pass this instance to a black-box algorithm for $X$, and finally, describe how to transform the output of the black-box subroutine to the final output.  A cartoon with boxes may be helpful.
\item
Prove that your reduction is correct.  As usual, correctness proofs for {NP}-hardness reductions usually have two components (“one for each f”).
\end{itemize}
\hrule
\bigskip

\newpage

\begin{enumerate}
%----------------------------------------------------------------------
\item Recall the facility location problem from HW 9. Prove that the
decision version of it is NP-Complete.
\item $k$-Color is the problem of deciding whether a given graph
$G=(V,E)$ can be colored with $k$ colors.
\begin{itemize}
\item {\bf Not to submit:} Describe a polynomial-time reduction
from 3-Color to 10-Color.
\item Describe a polynomial-time reduction 
from 10-Color to SAT. Why does this imply a polynomial-time reduction
from 10-Color to 3-Color?
\end{itemize}

\item Directed Hamiltonial-Path is the problem of deciding whether a
  given directed graph $G=(V,E)$ has a path that visits each vertex.
  Suppose you had black-box algorithm for Directed Hamiltonian-Path
  (note that this algorithm only answers YES or NO). Using the black
  box algorithm describe a polynomial-time algorithm that given a
  directed graph $G=(V,E)$ outputs a Hamiltonian Cycle in $G$ if it
  has one, or returns NO if it does not have any. Note that you are
  allowed to use the algorithm for Hamiltonian-Path more than once.
\end{enumerate}

\vspace{1in}

{\bf The remaining problems are for self study. Do \emph{NOT} submit for grading.}

\begin{itemize}
\item Reduce 3-SAT to 5-SAT. How does this generalize
  when you want to reduce 3-SAT to $k$-SAT where $k$ is some fixed constant?
  This is a useful exercise to understand the reduction from SAT to 3-SAT.
\item We briefly discussed in class how to reduce Dominating Set to
  Set Cover. Describe a polynomial time reduction from Set Cover to
  Dominating Set.
\item An instance of Subset Sum consists of $n$ non-negative integers
$a_1,a_2,\ldots,a_n$ and a target $B$. The goal is to decide if there
is a subset of the $n$ numbers whose sum is exactly $B$. The 2-Partition
problem is the following: given $n$ integers $a_1,a_2,\ldots,a_n$, is
there a subset $S$ such that the sum of the numbers in $S$ is equal
to $\frac{1}{2}\sum_{i=1}^n a_i$. It is easy to see that 2-Partition
reduces to Subset Sum. Do the reverse. Reduce Subset Sum to 2-Partition.

\item See HW 1 from Sariel's course in Fall 2015. 
\url{https://courses.engr.illinois.edu/cs473/fa2015/w/hw/hw_01.pdf}.

\item Jeff's notes, Kleinberg-Tardos and Dasgupta etal have several nice
problems on NP-Complete reductions. Skim through several of them to
quickly identify which problem you would use for the reduction.

\item See Jeff's home work 10 from Spring 2016.
last spring. \url{https://courses.engr.illinois.edu/cs473/sp2016/hw/hw10.pdf}
\end{itemize}
\end{document}

