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\noindent
\begin{center}
\framebox{
\vbox{\vspace{2mm}
\hbox to 6.28in { {\bf CS~151~~~Complexity Theory
\hfill Spring 2019} }
\vspace{4mm}
\hbox to 6.28in { {\Large \hfill Problem Set #1 \hfill} }
\vspace{2mm}
\hbox to 6.28in { {\it Out: #2 \hfill Due: #3} }
\vspace{2mm}}
}
\end{center}
\vspace*{4mm}
}
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% **** IF YOU WANT TO DEFINE ADDITIONAL MACROS FOR YOURSELF, PUT THEM HERE:
\begin{document}
\problemset{1}{April 4}{{\bf April 11}}
Reminder: you are encouraged to work in groups of two or three;
however you must turn in your own write-up and note with whom you
worked. You may consult the course notes and the text
(Papadimitriou). The full honor code and collaboration guidelines can be found in the course syllabus.
Please attempt all problems. {\bf To facilitate grading, please
turn in each problem on a separate sheet of paper and put your
name on each sheet. Do not staple the separate sheets.}
\begin{enumerate}
\item Downward self-reducibility. For a language $A$, define
\[A_{< n} = \{x \in A : \; |x| < n\}.\]
Language $A$ is said to be {\em downward self-reducible} if it is
possible to determine in polynomial time if $x \in A$ using the
results of queries of the form ``$y \in A_{<
|x|}$?'' The queries may be {\em adaptive}, meaning that the polynomial time procedure may choose later queries depending on the results of earlier ones. Show that every downward self-reducible language is in
{\bf PSPACE}.
\item Show that one of the following inequalities must hold:
$\mbox{\bf L} \ne \mbox{\bf P}$ or $\mbox{\bf P} \ne \mbox{\bf
PSPACE}$. Note that both are believed to be true, and no one knows
how to prove either one is true.
\item Show that logspace reductions are closed under composition.
Then use the same ideas to prove that if language $A$ is $\mbox{\bf P}$-complete,
then $A \in \mbox{\bf L}$ implies $\mbox{\bf L} = \mbox{\bf P}$.
\item Use a padding argument to show that if $\mbox{\bf L} =
\mbox{\bf P}$ then $\mbox{\bf PSPACE} = \mbox{\bf EXP}$.
\item Prove that $\mbox{\bf SPACE($O(n)$)} \ne \mbox{\bf P}$.
(Note that while this is an interesting result, it doesn't seem to
shed any light on the major open questions $\mbox{\bf
L}\stackrel{?}{=}\mbox{\bf P}$ and $\mbox{\bf
P}\stackrel{?}{=}\mbox{\bf PSPACE}$). Hint: consider a language
$A$ and a padded version of $A$. How are the two languages related
with respect to space? How are they related with respect to time?
\end{enumerate}
\end{document}